Yes, USB 3.0 Flash drive works on USB 2.0 and 3.0 ports, it works fine, got trial key and testing it now. I made no changes to BIOS config.
I have disconnected SSD drive that it has bad blocks, so maybe unRAID failed to mount it and somehow it caused fails for normal booting.
If it true, than you need to inspect booting of unRAID with bad blocks SATA disks, it not good if some bad disk can stop all system to work.