If your concern is booting USB for unraid the
solution is to sacrifice at least one of your drives
as a boot drive, or if you have the room get a pair
of small SAS drives and configure a raid 1 vol in
the perc controller.

AFAIK the USB is the dongle that unfair runs at all. You have to boot from it. After the boot its loaded into the RAM. But without the USB drive you cannot boot at all.

That is why I followed the proxmox route.