I just spent an entire night trying to reload my backup. I need legacy boot mode because previously I have experienced GPU passthrough not working in UEFI mode. However, the restored backup only works in UEFI but not in legacy mode.
HOW TO REPRODUCE
1. Download a usb stick backup from Main > Flash Device > Flash Backup
2. On another usb stick, restore the backup using Mac OS Unraid USB Creator (Version 1.8) Local Zip option, selecting the zip file from step 1
3. Attempt to boot the new flash drive in legacy mode
4. Error message: Not a bootable device
POTENTIAL WORKAROUND (Suggested by Frank1940)
Use the flash creator to load a brand new copy of 6.8.3. Afterwards, copy the content of the original /config folder from your backup.
If the USB Creator is not a reliable way to restore a backup until we find the cause, maybe we should update the documentation to mention this alternative method.
If you are a developer trying to reproduce this issue, I can send my previous backups that display this problem.