This would be a very simple feature to implement,
Allow mirroring of two USB UNRAID boot keys, this way if one fails there is an exact copy on the other that can simply be booted and the failed USB key rebuilt later from the web interface.
Simple MDADM raid I think would suffice, I intend to run UNRAID unattended, the motherboard has remote management interface including even viewing the BIOS over remote. Problem is I can't fix a failed USB key, but I could just boot a mirrored one.
.......... Actually, you know what would be even better than this?
An option in the web interface to "backup" to a spare USB key, that just uses DD to copy the entire disk to the backup usb key.
That way when we are happy with installations it's possible to take a backup of the boot system.