I've had no success booting from my T5 formatted as APFS, even after re-installing Mojave on it. So I erased the drive, formatted it as HFS+, and tried again, still with no luck. The boot process hangs about half-way through consistently. I thought it was the drive, so I tried booting from another APFS-formatted T5, and that didn't work either. So I'm wondering if it's my iMac. I'll try both drives with my MacBook Pro next.I have a small business with a dozen iMacs, circa 2014-2017, all booting off external T5's, some formatted as APFS with Disk Utility, some auto-formatted to APFS via Mojave installer, all running Mojave. All work fine.