M1 Mac cannot boot from external drive with FileVault enabled

Number:rdar://FB9037296 Date Originated:3/9/2021
Status:Open Resolved:
Product:MacBook Air Product Version:M1, 2020
Classification: Reproducible:Yes
Summary: When macOS 11.2.2 is installed on an external Thunderbolt 3 SSD, the Mac can boot from it properly, but only if FileVault is disabled. Once FileVault is enabled on the external drive, the Mac cannot boot and gets into a restart loop.

Steps to reproduce:
- Attach a Thunderbolt 3 SSD.
- Using Disk Utility, erase the external SSD as APFS/GUID Partition Map.
- Download Big Sur 11.2.2 installer and run it, with the external SSD as the destination.
- Install a fresh copy of macOS on the external SSD, all defaults, no migration.
- At this point, the Mac can restart from the external SSD, and all is well.
- Now, while booted from the external SSD, enable FileVault. Wait until encryption finishes.
- Restart.
- The Apple logo appears and the progress bar begins to fill, but then the Mac restarts in recovery mode. If I then select the external SSD as the startup volume, the cycle repeats. At no point does the login window appear, nor am I prompted to unlock the external FileVault volume.

Expected behavior:
- When an external SSD with a properly installed copy of macOS is installed, with FileVault enabled, and that volume is selected as the startup volume, the Mac should in fact complete the startup process.


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