... I tried to transfer my mail over to the spare. I'm still using Apple Mail (yes, a mistake). In older versions, copying ~/Library/Mail successfully transferred my account settings and mail database. No more. When starting Mail on the spare, It came up as if it had never been run and wanted me to set up from scratch. The account settings have been moved into System Preferences > Internet Accounts. I could not find where the settings are actually stored, so I tried recreating the accounts on the spare. It still won't use my mail database I copied over. Fail #2.
My conclusion: Apple's apps are too closely married to the OS. Recovery is far more difficult now than in previous Mac OS X versions. Thanks, Apple.
- Apple Mail, especially on OS X, has been problematic in some versions.
- Apple Mail, especially on macOS, has been quite good for many users.
- Apple Mail, for many versions of OS X and macOS, has become entwined with iCloud and Mail on iOS. Trying to transfer messages or rules using file copy has long been almost guaranteed of failure.
- Since Apple Mail has become entwined with iCloud and Mail on iOS, Migration Assistant has had good success probability. Just not always perfect.
- Long ago it became obvious that an intermediate IMAP server was the best medium for wholesale transfers of messages between instances of Mail User Agents (MUAs) or Servers (MTAs), including Apple Mail and Microsoft Outlook. I actually created my own local IMAP server for this before iCloud was born to use as a migration tool from the Windows world.
- Recently, I have used an iCloud account with sufficient storage as an intermediate server for non-iCloud email accounts. This works for both local and account mail folders.
- Create a new iCloud folder
- Drag whatever folders need to be transferred to that folder
- Wait a long time.
- On the target machine, drag the folders to you desired locations.
- Clean up appropriately.
- The bottom line for this is that IMAP accounts do not require any manual transfer processes for messages and can be used as intermediates for transferring On My Mac mailboxes.
As an aside, my personal macOS Apple Mail has one iCloud account, one mail.com account, two Gmail accounts, and two comcast.net accounts. This fluctuates as additional accounts are added for test purposes, as in verification and diagnosis of a client's email problems. The base set has survived intact through many OS X and macOS in-place updates, migration to new OS X and macOS instances. Since they are IMAP-based, even activating one or more mail accounts on older macOS versions (Snow Leopard Server, Sierra, and High Sierra) is relatively simple.
I use iCloud Safari to synchronize bookmarks plus a third party to copy these to Firefox. I refuse to share macOS keychains in iCloud. And, I keep lots of stuff in 1Password (in AgilbitsCloud) and Dropbox.
Time Machine and Carbon Copy Cloner are my go-to backup and restore applications.