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APFS / file systems

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Ric Ford

MacInTouch
I'm going to start this new thread to cover Apple's new APFS file system (Apple File System) and related topics, because it's extremely important. Why? Because everything you do on your computer depends on the file system! If that's buggy, it's very bad news. If it's slow, you suffer. If it can't handle security, or can't handle it properly, that's a giant issue.

We've already talked about some of these things in other topics, and I probably can't incorporate all of those here, but I'll try to pull together some links and notes from them, while adding new material going forward.

Some starting points:
 


Ric Ford

MacInTouch
Some observations from a scan through the Apple File System Reference PDF:
  • Despite being late and incomplete, the document has useful information and background, noting, however:
    • Most apps interact with the file system using high-level interfaces provided by Foundation, which means most developers donʼt need to read this document. This document is for developers of software that interacts with the file system directly, without using any frameworks or the operating system—for example, a disk recovery utility or an implementation of Apple File System on another platform.
  • An APFS partition can boot a Mac via its 'embedded EFI driver.'
  • There's a container flag for 'uses software cryptography'.
  • There are volume flags for encryption, including one for effaceable storage.
  • There's a compatibility flag for 'container supports Fusion Drives'.
  • There's a flag for 'container is using low-capacity Fusion Drive mode'
  • There are flags for defragmentation support.
  • APFS Version 1, implemented in macOS 10.12, is incompatible with the Version 2 of macOS 10.13.
  • Encryption descriptions are absent: "no overview available".
  • Ditto for Fusion: “no overview available".
 


Thank you, that explains behaviour I'd noticed - I needed to free up extra space in order to install Mojave, and rebooted (to clear caches etc). I noticed that for a few minutes after rebooting, the free space figure kept increasing and increasing. I was worried for a moment that there was a problem, but then the figure stabilized. I guess it was just this 'processing file deletes' process.
I now believe that APFS is similar to WAFL [Write Anywhere File Layout], due to the way it behaves:

 


I had recently run into a situation where a process was rapidly eating into my disk space (formatted as APFS), and as I noticed it, I began surfing the disk for files to delete before the volume ran out completely. I couldn't do it. The disk ran out of space completely. At this point, APFS would not allow me to do anything, and that includes delete any more stuff.
... Fortunately, when the disk space ran out, the process that was eating disk space failed. And allowing the computer to just sit there, APFS began processing in the background the files I deleted in the mad rush to delete stuff. Space just began to appear, and all was well.
Thank you, that explains behaviour I'd noticed - I needed to free up extra space in order to install Mojave, and rebooted (to clear caches etc). I noticed that for a few minutes after rebooting, the free space figure kept increasing and increasing. I was worried for a moment that there was a problem, but then the figure stabilized. I guess it was just this 'processing file deletes' process.
It sounds like we may be seeing the effect of snapshots at play here.

If a snapshot is created, and then you delete files, the free space won't increase, because the blocks are being held by the snapshot. When the snapshot is deleted, the blocks will be freed (unless they are being used by other snapshots, of course).

I don't know exactly what macOS is doing here, but if there is something set up to create periodic snapshots (e.g. a Time Machine feature, perhaps), then that would explain what you observed. You were deleting files that were also in snapshots. When space ran out (or hopefully some time before that), the OS started deleting older snapshots, in order to free up space, which is when you saw the free space increase.

I've seen this exact behavior on other systems that use snapshots for local backups (e.g. NetApp file servers).
 


Ric Ford

MacInTouch
Apple's instructions regarding APFS vs. HFS+ (a.k.a. "Mac OS Extended"):
Apple Support said:
How to erase a disk for Mac
...
Which format to use: APFS or Mac OS Extended
Disk Utility in macOS High Sierra or later can erase most disks and volumes using either the newer APFS (Apple File System) format or the older Mac OS Extended format, and it automatically chooses a compatible format for you. If you want to change the format, answer these questions:

Are you formatting the disk that came built into your Mac?
If the built-in disk came APFS-formatted, don't change it to Mac OS Extended.

Are you about to install macOS High Sierra or later on the disk?
If you need to erase your disk before installing High Sierra or later for the first time on that disk, choose Mac OS Extended (Journaled). During installation, the macOS installer decides whether to automatically convert to APFS—without erasing your files:
  • macOS Mojave: The installer converts from Mac OS Extended to APFS.
  • macOS High Sierra: The installer converts from Mac OS Extended to APFS only if the volume is on an SSD or other all-flash storage device. Fusion Drives and traditional hard disk drives (HDDs) aren't converted.
Are you preparing a Time Machine backup disk or bootable installer?
Choose Mac OS Extended (Journaled) for any disk that you plan to use with Time Machine or as a bootable installer.

Will you be using the disk with another Mac?
If the other Mac isn't using High Sierra or later, choose Mac OS Extended (Journaled). Earlier versions of macOS don't mount APFS-formatted volumes.
 



For several reasons, I decided to set up a 2018 13" MacBook Pro with the internal drive partitioned as GUID / HFS+ Journaled. This was accomplished by booting from an external drive with macOS 10.13.6 already installed and then firing up Disk Utility. I was warned that APFS was the optimized default, but I was allowed to proceed. There was also a warning that one or more security features depended on APFS (I believe something to do with encryption?), but I moved ahead. Carbon Copy Cloner successfully cloned over from the external drive, including the Recovery partition.
I have a similar new MacBook Pro 13” inbound, and I am also considering reverting it to HFS+ before I really start using it. Can you please clarify the procedure you used? How did you create the external with macOS 10.13.6? Was it a clone of your internal, and was it HFS+ or APFS?

Are you using FileVault since doing your conversion, or was your encryption warning regarding the use of FileVault?

I was thinking about starting the new machine in target disk mode, even before doing a normal start and setup, and making a clone of it.
 


I have a similar new MacBook Pro 13” inbound, and I am also considering reverting it to HFS+ before I really start using it. Can you please clarify the procedure you used?
1. On the new laptop, fully update macOS 10.13.6 with latest App Store updates, install all 3rd-party apps, and customize settings/preferences as desired, copy over all personal data from a current backup, and confirm all is working exactly as you expect.​
2. Format an external hard drive or SSD as GUID/HFS+.​
3. Use Carbon Copy Cloner to clone the entire MacBook Pro 13" drive to the external drive. Make sure you take advantage of CCC's offer to create the Rescue/Recovery partition at the end of the clone procedure.​
4. Boot from the external.​
5. Fire up Disk Utility on the external and reformat the MacBook Pro 13" internal drive as GUID/HFS+. Ignore the warnings about abandoning APFS.​
6. Use CCC to clone from the external back to the internal. Again, make sure to create the Rescue/Recovery partition. CCC makes this dead simple (unlike SuperDuper).​
7. Boot from the internal.​
8. There is no step 8! You're done.​
And no; I was not / am not using FileVault. I would suggest disabling it prior to starting this procedure.
 


Ric Ford

MacInTouch
Tim Standing gave another presentation at the most recent MacSysAdmin conference, and his talk, while technical, is well worth watching for details about APFS, performance issues, the T2 system (and Apple flash storage), Apple bugs and developer support, and more... (including a Q&A session at the end):
Jonathan Levin went deeper into APFS technical details following Tim Standing's presentation at the same conference, using some very interesting tools for analysis:
Jonathan also has a very interesting presentation about Apple's T1 and T2 systems (incorporated in Apple's 2016+ MacBook Pro and iMac Pro models):
 


I now believe that APFS is similar to WAFL [Write Anywhere File Layout], due to the way it behaves:
I'm not a file system geek, but, according to what I'm reading about APFS (and through Ron's link), it appears that this sort of file writing technique is living life on the edge without some sort of a hardware RAID system that utilizes parity-based error correction.

If I'm wrong, it won't be the first time, but I'm sticking with Sierra and HFS+ for my production machines. My MacBook Pro has a Mojave partition for testing, but it's HFS+.
 


Ric Ford

MacInTouch
I'm not a file system geek, but, according to what I'm reading about APFS (and through Ron's link), it appears that this sort of file writing technique is living life on the edge without some sort of a hardware RAID system that utilizes parity-based error correction. If I'm wrong, it won't be the first time, but I'm sticking with Sierra and HFS+ for my production machines. My MacBook Pro has a Mojave partition for testing, but it's HFS+.
Tim Standing's presentations (linked above) have some enlightening perspective on errors and error correction, including Apple kernel / Thunderbolt bugs, non-ECC memory errors, differences in APFS vs. HFS metadata integrity, etc. APFS does have some advantages for data integrity, although its constant changes are a concern regarding potential bugs that may not yet be discovered.
 



APFS cautionary tale:

As a musician, I have sample libraries, some on the internal drive but most on external drives.

When I bought a 2017 touch-bar MacBook Pro (now on High Sierra), I ordered it with a 2TB boot drive. I wanted to move some of my libraries to the internal drive. Rather than re-install them all, it was easier to partition into two 1TB partitions with the second one being named the same as an older external drive where those libraries were.

Gradually I began to fill up both partitions, so I bought a newer, bigger external drive (4TB Samsung SSD). My plan was to erase and remove the second partition on my boot drive, returning it to its full 2 TB capacity, then copy some of the libraries back to it but leave most on the external drive.

I opened up Disk Utility, erased the second partition (which was HFS+) and then clicked the minus button to reclaim that space. My computer froze and would not respond to any key strokes.

Somehow, the warning message that that would occur had gotten hidden, and i did not see it. After about 10 minutes, I figured it was crashed and force restarted. But the boot drive was still 1 TB. So I tried to delete the second partition again. This time, I got the warning message, but then an error that the operation could not be completed.

Now my system was in a very weird state - If I tried to erase the second partition, it actually created a volume onto the first partition, but with zero space on it. That is, it appeared as two 1 TB volumes, but the second one had no free space at all.

At first I thought I might have to reformat the internal hard drive and reinstall from the network - but I'm not sure that even works on these laptops, because you will get Sierra, not High Sierra. I also remember reading somewhere that the touchbar requires a hidden partition, and if you erase it, you may not be able to start up at all.

In any case, I finally found the solution. I booted from an external backup and, in Terminal, (typing diskutil list) found the disk identifier of the "Apple_APFS Container disk1", which was disk0s2. Then I was able to resize it:

diskutil apfs resizeContainer disk0s2 0

The "0" tells it to use all available space.

My machine did not freeze - I assume the freeze only happens if you are trying to resize the partition on the drive you are booted from. It was quite a journey.
 


I'm thinking of moving to Mojave from High Sierra, but all my drives, including the boot drive which is a 2TB Fusion drive, are formatted as Journaled HFS+. The external drives (9 TB on 4 drives) are all rotating drives.

I don't want any of the drives reformatted to APFS if I install Mojave. Is that possible? If it is, what functional problems might I encounter? Will the next OS require APFS formatting?
 


I'm thinking of moving to Mojave from High Sierra, but all my drives, including the boot drive which is a 2TB Fusion drive, are formatted as Journaled HFS+. The external drives (9 TB on 4 drives) are all rotating drives.

I don't want any of the drives reformatted to APFS if I install Mojave. Is that possible? If it is, what functional problems might I encounter? Will the next OS require APFS formatting?
If you invoke the installer from the shell:

/Applications/Install\ macOS\ Mojave.app/Contents/Resources/startosinstall --converttoapfs NO

Disclaimer: I haven't tried this myself, and the original article was for High Sierra.
 



FWIW: I recently did the update from High Sierra to Mojave (not a clean install), and only the boot Crucial 512GB SSD was converted to APFS. All four spinning 4TB drives in the Mac Pro bays remained HFS+.
 


APFS cautionary tale:
...
I opened up Disk Utility, erased the second partition (which was HFS+) and then clicked the minus button to reclaim that space. My computer froze and would not respond to any key strokes.
...
In any case, I finally found the solution. I booted from an external backup.
....
My machine did not freeze - I assume the freeze only happens if you are trying to resize the partition on the drive you are booted from. It was quite a journey.
I'm not sure that is an APFS issue specifically. Trying to resize a partition that you are actively booted into and using is risky. There are corner cases where a complete "undo" of a partition scheme will work, but generally is cleaner to stop actively using all of the partitions that you are trying to mutate and to have a clean back-up of any partition contents you want to have afterwards.

The standard diskutil resizeVolume operator will 'unbless' a boot volume. It is an APFS resize verb being used here but indicative of the interaction between boot and partition resizing (verbs needed to complete the task at the low level).
 


Here's a description of the options for High Sierra - I don't know if this applies to macOS 10.14 Mojave:
How to prevent conversion to APFS on High Sierra install
Sadly, this option no longer works in Mojave. The only known method that I am aware of is to load Mojave onto an external spinning drive formatted as HFS+ and then clone that back to your internal SSD, though I have not tested that myself.

Here's hoping an alternative method surfaces by the time I am ready to start using Mojave on my main Mac (which would probably be around macOS 10.14.3).
 


There is another possibility to install Mojave without automatic conversion into APFS. I refered to this as the "dude" method in my other post from today.
 


Sadly, this option no longer works in Mojave. The only known method that I am aware of is to load Mojave onto an external spinning drive formatted as HFS+ and then clone that back to your internal SSD, though I have not tested that myself. Here's hoping an alternative method surfaces by the time I am ready to start using Mojave on my main Mac (which would probably be around macOS 10.14.3).
I do not understand the reluctance with APFS, because in my case, it has worked well as far as I know since the first day it was installed, I think, in the middle of Sierra. I did, however, make a clone of my machine before the upgrade to APFS. In my case, the user computer does not have important files - all of the data files are on my server. Thus, I would not lose anything important if I had to start over with any current OS. So instead of fighting APFS, I am using the Carbon Copy Cloner clones (I have more than one) to back up in case something goes bad. That is also why I am really rather cavalier about using beta versions. I suspect, for the most part, the beta versions are bug fixes, and why wait for the next dot upgrade to get the current bug fixes?
 


1. On the new laptop, fully update macOS 10.13.6 with latest App Store updates, install all 3rd-party apps, and customize settings/preferences as desired, copy over all personal data from a current backup, and confirm all is working exactly as you expect.​
2. Format an external hard drive or SSD as GUID/HFS+.​
3. Use Carbon Copy Cloner to clone the entire MacBook Pro 13" drive to the external drive. Make sure you take advantage of CCC's offer to create the Rescue/Recovery partition at the end of the clone procedure.​
4. Boot from the external.​
5. Fire up Disk Utility on the external and reformat the MacBook Pro 13" internal drive as GUID/HFS+. Ignore the warnings about abandoning APFS.​
6. Use CCC to clone from the external back to the internal. Again, make sure to create the Rescue/Recovery partition. CCC makes this dead simple (unlike SuperDuper).​
7. Boot from the internal.​
8. There is no step 8! You're done.​
And no; I was not / am not using FileVault. I would suggest disabling it prior to starting this procedure.
Thanks for the detailed steps. I got up to the point of the warning you previously mentioned, and it was pretty vague, something about reduced privacy. (I was hoping for something more enlightening about encryption or FileVault.)

Since I do want to have FileVault enabled in the end, I did some more searching, including reading some items in the Carbon Copy Cloner support area. I found this comment there:
Our testing has confirmed that Macs with Apple's T2 controller chip cannot boot from an encrypted, "Mac OS Extended"-formatted, external volume.
I posted a question about whether they knew if this also applied to internal drives, and Mike replied, saying that they hadn't tested it but suspected the same limitation would apply to internals as well.

So, I guess I will try to live with APFS for now, while all our other Macs remain at Sierra or earlier.
 


I do not understand the reluctance with APFS, because in my case, it has worked well as far as I know ...
Stable or not, it is a real problem that detailed specifications are unknown outside of Apple. This means there are no third-party disk utilities (e.g. DiskWarrior, TechTool Pro, etc.) that can repair/optimize an APFS volume. If something gets damaged beyond the ability of Apple's bundled tools to fix, you are left with no option other than to wipe the volume and restore from a backup.

For many people, this is an unacceptable position. Even if you have current backups (as everybody should), such a restore is extremely time-consuming. Until a good suite of repair/maintenance tools is available, many people are not going to want to switch.
 


Stable or not, it is a real problem that detailed specifications are unknown outside of Apple. This means there are no third-party disk utilities (e.g. DiskWarrior, TechTool Pro, etc.) that can repair/optimize an APFS volume. If something gets damaged beyond the ability of Apple's bundled tools to fix, you are left with no option other than to wipe the volume and restore from a backup.

For many people, this is an unacceptable position. Even if you have current backups (as everybody should), such a restore is extremely time-consuming. Until a good suite of repair/maintenance tools is available, many people are not going to want to switch.
My comment is that I have not used a disk utility to repair anything that I own in several years or more. (I have on occasion used DiskWarrior to repair a friend’s older hard disk.) I have DiskWarrior and have used it often many years ago, particularly with spinning hard disks. I used to repair permissions often using DiskWarrior, and I have an external hard disk called Troubleshooting, which allows me to boot and run diagnostic software. I have not needed the troubleshooting hard disk in years, even though I keep it updated on two partitions.
 


Ric Ford

MacInTouch
Howard Oakley looks at fusion drives in APFS vs. HFS+ and continuing confusions created by Apple's Disk Utility app and invisible partitions:
The Eclectic Light Co. said:
Fusion Drives in APFS
macOS has actually supported Apple’s new file system APFS on Fusion Drives for well over a year: beta versions of High Sierra could convert Fusion Drives to its version 2 APFS between June-September 2017, but this support was pulled at the last minute from High Sierra’s release version. When you now upgrade a Mac with a startup Fusion Drive to Mojave, that drive will be converted to APFS; you don’t have an option.
...
There are, though, two traps for those who need to refer to volumes by their disk identifiers: with a single-device APFS drive, synthesised volumes are partitions of /dev/disk1, but on a Fusion Drive they are partitions of /dev/disk2; designations in APFS are very different from those in HFS+.
...
Disk Utility’s simplified view shows only /dev/disk2, the Fusion Drive as a whole. At the top level, it confirms that it has a standard GUID Partition Map, and claims the capacity of the total of its two components.

The container-level view shows disk2s1 and disk2s4 in reverse order, but doesn’t show the pre-boot or Recovery volumes, which could be confusing. Instead, it reports 2 Not Mounted, presumably disk2s2 and disk2s3, which are here claimed to amount to nearly 10 GB, and a spurious Free, which isn’t a volume at all.

Finally, its volume view shows more than just disk2s1, and is really the current allocations within the /dev/disk2 container. I find this confusing, particularly in comparison to the diskutil apfs list results shown above.
 


Ric Ford

MacInTouch
OK, so I just ran this test:

I split a Samsung 970 EVO (installed in an OWC Express 4M2) into two partitions), one HFS+ and one APFS. I then cloned a basic High Sierra system setup onto both.

Booting from APFS introduces an annoying delay, which I've seen on multiple systems and multiple installations, roughly around "70%" of the way through Apple's hidden boot processes while only a progress bar is displayed on the screen.

If I boot a clone of that same macOS installation from the HFS+ volume, there's no delay.
 




OK, so I just ran this test:

I split a Samsung 970 EVO (installed in an OWC Express 4M2) into two partitions), one HFS+ and one APFS. I then cloned a basic High Sierra system setup onto both.

Booting from APFS introduces an annoying delay, which I've seen on multiple systems and multiple installations, roughly around "70%" of the way through Apple's hidden boot processes while only a progress bar is displayed on the screen.

If I boot a clone of that same macOS installation from the HFS+ volume, there's no delay.
I wonder if this is causing the startup delay on your APFS volume?
Bombich.com said:

  • The Startup Security Utility reports that authentication is needed, but no administrators can be found
  • After cloning to an APFS volume that previously had FileVault enabled, the destination can't be unlocked on startup
  • After cloning to an APFS Encrypted volume there is a 24-second stall during startup
All of these conditions are caused by the same underlying problem: users on the affected volume do not have access to the volume's Secure Token. There are generally two ways to get to this result:
  • The volume was erased as an encrypted volume, thus no user account was associated with the unlocking of that volume, or
  • The user accounts that are allowed to unlock the disk belonged to some previous installation of macOS on that volume
Solution: Erase the destination in Disk Utility before proceeding with the cloning task. You should erase the destination as "APFS", not "APFS (Encrypted)". For more technical users, we offer some additional background information below.
 


Ric Ford

MacInTouch


I wonder if this is causing the startup delay on your APFS volume?
I will have to try to understand the Bombich link. I cloned my internal disk to an unencrypted volume, as he recommends, but encrypting after that did not go well.

I tried to set up my new Mac Mini to use only the external Samsung T5 disk.
1. Carbon Copy Cloner copied the internal drive to the T5.​
2. Installed all my applications.​
3. Manually copied all my files.​
Everything seemed to work just fine for a day, and I kept the internal drive unmounted. Then I turned on FileVault. Took me most of today to recover from that. If anyone knows what I did wrong, if I did, please enlighten me. It is a long story but I hope there is enough detail to mean something to someone.

Turning on Filevault required restart. I said “fine” and restarted - and my day came crashing down. The Mini started to boot, the white line got part way along, and the white line started over. It got further along, and it started over again. It got part way along and then I got an error message that it can’t verify the disk.

Repeated attempts had the same result, so I booted back to the internal drive. I tried to run Disk Utility First Aid on the T5. It passed the first few steps and then reported it could not unmount the disk to continue. I tried to unmount it manually with Disk Utility, and that failed. I right-clicked on the desktop icon, and it unmounted immediately. Disk Utility refused to mount it again. It said the disk was encrypted.

I used diskutil which showed steady encryption progress. Took nearly an hour to complete, though the percentage rate was slow during the middle and fast at the beginning and end. The volume was not mounted, so this surprised me.

It appears FileVault can be turned on/off only for the boot disk, at least from the System Preferences > Security tab. The command line fdesetup has a -device option to specify a volume, but the only commands that seem to have that option are checking to see if a recovery key exists.

Once the encryption process completed, Disk Utility would mount and unmount the T5, and First Aid said everything was fine. It seems Disk Utility and encryption-in-progress are not compatible. I don’t have another setup to check that. This may be the only useful information I gained from this long tale.

Bad news. Still would not boot to the Samsung T5. At this point, I figured the disk is a loss, since I can’t boot from it, and I can’t disable FileVault, so I am free to try anything at all.

I went back into Startup Security and changed it to Medium Security for any Apple OS and tried again. Same slow startup, but it went into Reinstall OS with no way out. Didn’t want that.

I powered it off and restarted back into Startup Security and set it to No Security. Slow startup, and this time it claimed a software update ie required to use the Samsung T5, so I told it to update. A few seconds later it said “An error occurred installing the update” and let me Try Again (which immediately returned me to the same error screen) or choose another startup disk.

I tried to set it back to Full Security, and it refused, saying “Internet connection required”. I have to “select a disk to boot and security information for that drive has to be downloaded from Apple”. Too bad there isn’t any way to select a disk or get an internet connection, even though I added an ethernet cable. I was able to set it back to Medium Security.

I restarted, and Startup Security showed it back to Full Security. I didn’t set that. I wonder if it remembered I had tried, and the successful boot reset it for me? That seems like a really bad thing to do, overriding my security choices without warning. I tried to boot from the Samsung T5 again, but after a slow process of something, it said I had to boot from the internal drive.

I set it back to Medium Security to see if the Install OS option came up again. It did, so I tried to install. It said it could not contact the recovery server. Even though Wifi is on, I discovered I had to manually select a network using that almost invisibly gray icon at the upper right corner of the screen. Tried again after joining the network, and it still said it could not contact the recovery server. The bad news at this point is there is no menu bar, just the macOS Utilities window. Selecting any option in the window does not activate the Continue button, but trying to close the window let me choose a startup disk and restart.

I could access the Samsung T5 files when booted from internal disk, but the T5 refused to boot. I gave up and erased the drive, leaving it as an empty encrypted APFS container. Downloaded Mojave and installed it, and it finally went well. Final verdict: encryption after install prevented booting, but encrypting before install worked. Is FileVault the same as initially creating the APFS container as encrypted?

Ric, in your Mac Mini posts, one place you said your Samsung T5 was encrypted, and in another place you said you booted from it, but you didn’t specifically say you managed to boot from the encrypted T5. If you did, how did you encrypt it - before or after installing the OS?

Last bit: I am still trying to figure out how to keep the Mini from trying to mount the internal disk when I boot to the Samsung T5. I can hit Cancel instead of providing the disk password, but that lacks elegance. I tried a few variations of /etc/fstab settings, but none helped.
 


Ric Ford

MacInTouch
... Ric, in your Mac Mini posts, one place you said your Samsung T5 was encrypted, and in another place you said you booted from it, but you didn’t specifically say you managed to boot from the encrypted T5. If you did, how did you encrypt it - before or after installing the OS?
My standard practice is to encrypt a volume before putting anything on it, including macOS. This may be part of the severe problems I also had with macOS updates/installs trying to get the High Sierra macOS on the 2018 Mac Mini updated (which I described previously). I had to wipe the Mac Mini drive clean in Recovery mode and use the special key combination that installs Mojave from the Internet. Things have been working since then.
Last bit: I am still trying to figure out how to keep the Mini from trying to mount the internal disk when I boot to the Samsung T5. I can hit Cancel instead of providing the disk password, but that lacks elegance. I tried a few variations of /etc/fstab settings, but none helped.
I am seeking a solution to the same problem. Please keep us posted if you find a good one. (I suppose it's possible to write a shell script or AppleScript to do this, but I haven't spent hours trying to sort that out yet.)
 


… Bad news. Still would not boot to the Samsung T5. At this point, I figured the disk is a loss, since I can’t boot from it, and I can’t disable FileVault, so I am free to try anything at all.
At any point, did you try to run the bless terminal command?

(bless can also be the solution to preventing a volume from mounting at startup, although I never tried that command yet.)
 


At any point, did you try to run the bless terminal command?
(bless can also be the solution to preventing a volume from mounting at startup, although I never tried that command yet.)
No, bless never occurred to me. I assumed that if it had been booting, it would continue to boot, and when it didn't, I went down a rabbit hole. From my reading, the bless command basically points out where the boot process begins so the computer knows where to start. I need the internal disk to be bootable, just not to auto-mount.
 


OK, so I just ran this test:

I split a Samsung 970 EVO (installed in an OWC Express 4M2) into two partitions), one HFS+ and one APFS. I then cloned a basic High Sierra system setup onto both.

Booting from APFS introduces an annoying delay, which I've seen on multiple systems and multiple installations, roughly around "70%" of the way through Apple's hidden boot processes while only a progress bar is displayed on the screen.

If I boot a clone of that same macOS installation from the HFS+ volume, there's no delay.
Is it a 24-second delay, as Mike Bombich noted? It's a few seconds on my system-- not 24, which would have been intolerable. However, there were some differences with respect to the instructions: I just migrated my system from a backup using Migration Assistant. What I noted was that there was no delay initially, when the volume was APFS unencrypted, but the delay was introduced. as soon as I enabled FileVault encryption.
 


Last bit: I am still trying to figure out how to keep the Mini from trying to mount the internal disk when I boot to the Samsung T5. I can hit Cancel instead of providing the disk password, but that lacks elegance. I tried a few variations of /etc/fstab settings, but none helped.
I am seeking a solution to the same problem. Please keep us posted if you find a good one. (I suppose it's possible to write a shell script or AppleScript to do this, but I haven't spent hours trying to sort that out yet.)
I just verified this method works successfully with Mojave. I tested it with an external HFS+ hard drive.
In addition to those instructions, here is how to remove a volume you added to that list.
  1. Follow steps six and seven as described above.
  2. Next, use the arrow keys to move the cursor to the line you created in step eight.
  3. Type dd (lowercase) to remove that entire line.
  4. Follow steps nine through eleven as described above.
 



I just did this, using macOS Sierra 10.12.6 and tried to prevent a couple of volumes on the internal Apple flash drive from mounting by setting up the fstab file as directed. Unfortunately, it didn't work on a reboot - the volumes still mounted.
I had the same experience - the fstab file setting doesn't work on my internal drive. (I tried two or three variations I found recommended at various places.) I found one posting somewhere that said this no longer works with APFS but didn't supply any information about how they deduced this. I talked to second-level Apple Support, and she said this may be related to it being an internal drive, but she didn't have any solid information either. She promised to try to find out how to do this....
 


I just did this, using macOS Sierra 10.12.6 and tried to prevent a couple of volumes on the internal Apple flash drive from mounting by setting up the fstab file as directed. Unfortunately, it didn't work on a reboot - the volumes still mounted.
After safely unmounting all currently mounted volumes,
sudo chflags uchg /Volumes
will prevent any volumes (other than your startup volume, which mounts at /) from mounting.

Once you've figured out the magic words (I'm equally puzzled...) to prevent certain volumes from mounting (whether with the fstab file or some other wizardry)
chflags nouchg /Volumes
will re-permit normal mounting at startup/whenever.
 


I just did this, using macOS Sierra 10.12.6 and tried to prevent a couple of volumes on the internal Apple flash drive from mounting by setting up the fstab file as directed. Unfortunately, it didn't work on a reboot - the volumes still mounted.
I tried it out this afternoon on a 17" iMac 5,1 running Snow Leopard 10.6.8, with the stock Apple hard disk drive. Also on a 2012 Mac Mini i5 running High Sierra 10.13.6, with an internal Samsung 840 EVO SSD. Both machines have their storage split into two HFS+ partitions. It worked on both of the machines. I also verified Carbon Copy Cloner successfully mounted the Mac Mini volume when running its backup task.

I don't have a Mac with stock Apple flash, or a Sierra 10.12 startup disk, or use FileVault, so those may be relevant variables.
 


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