I know your saying it doesn ' t matter but to me it points to the USB being shuffled out of the boot order as well . I ' ve booted just fine with 23 bays filled with drives 2 TB . Even with the 24 th bay filled it would still boot , just the 24 th drive didn ' t show up in unRAID . This may sound silly but you could always boot with whatever Drive amount it will actually boot with , the once in the unRAID GUI add the other two drives back in and try a reboot . I know that sounds stupid but it ' s worth a shot . This was years ago when I had a similar issue and it was a bios issue . Had to go in to the old . very old . bios and tell it the USB was a hard drive or something like that and it would finally boot correctly . All of my newer boards booted correctly .