I tried to restore from a Seedvault backup on a USB flash drive, but the system did not recognize the drive at all!
- First, I was using a bad USB-C to USB-A adapter. That is a common issue. I tested this on my computer, and it was not recognized. After throwing that one away, I tested the USB adapter included with the phone, and that worked on the computer.
- Even after using a good USB adapter, the phone did not recognize the USB drive after setup. I found the USB Settings menu and selected "This Device" under "USB Controlled By", since it looked like this was trying to interface with my USB flash drive as if the flash drive was a host (which would be incorrect). When I tried to change I got an error "couldn't switch" and the radio button stayed at "Connected Device." I spent awhile chasing this error down, only to find that it seemingly did not apply. Each attempt at this would disable the radio buttons, so I would have to restart the phone to get another try. Attempting to make this change would also cause the "Storage" menu in Settings not to see the USB flash drive anymore.
- I found the "Storage" menu in Settings one time after rebooting, and this saw the flash drive! But it said it was "corrupted." The first part of the drive contained an iso9660 filesystem and rest of it was ext4-formatted. Unfortunately, it seems like Android only supports FAT32 and exFAT. I just had the Storage menu "fix" the flash drive, and that formatted it to a single FAT32 partition.
- I re-copied the Seedvault backups onto the FAT32 partition, but upon inserting it, GrapheneOS setup did not see a backup. I was backing up my .SeedVaultAndroidBackup folder with Syncthing, but it turns out that just copying the contents of that folder to a flash drive is not sufficient to restore from it as a backup. On a computer, I created a new folder ".SeedVaultAndroidBackup" on the flash drive and moved all of the Seedvault files into that folder. After re-inserting the flash drive into the phone, GrapheneOS setup correctly identified my backups and started the restore process.
It seems like a lot of things went wrong for me during this process, so hopefully this helps some people!
Good luck.
-Nicholas