Big Sur Upgrade Loop
When upgrading to Big Sur, some users[1] may get stuck in a loop in Boot Recovery Assistant where the user has two options:
- "Select a user you know the password for:"
- No passwords work (normal login password, Apple ID password, 1234, [blank], etc.)
- "Forgot all passwords?"
- Using Apple ID/password to attempt to reset the user account password or using FileVault recovery key fails (Message says "The supplied account was unable to unlock this volume.") and only gives the option to restart the computer, restarting the loop.
To get around the loop, the best way (so far) is to:
- Hook up the Target computer (ie the one in the loop) in Target Disk Mode to a Host computer running High Sierra or Mojave[2].
- Clone the data partition to a temporary drive (e.g. with Carbon Copy Cloner).
- Erase the Target computer.
- Install the desired version of macOS.
- During macOS setup, migrate the data back from the temporary drive.
[Note 1: Initial impressions from the couple cases I've run into are that it seems to happen when the user tries to upgrade when they have little to no free space on their drive, causing the upgrade to crash, leading to the computer loading into Boot Recovery Assistant with an error that says "An error occured preparing the software update." and showing the startup disk selection menu where there are no disks available to choose to startup from, only leaving the user the options to restart or shut down, where restarting causes the loop.]
[Note 2: A Host machine running Catalina or Big Sur will not work; when attempting to mount the Target computer on Catalina or Big Sur, no matter what you type for the FileVault password it will not accept it, whereas it will accept it on High Sierra or Mojave.]