Hi,
It sounds like you were experiencing an issue very similar to mine I had a few months ago where Unraid will occasionally boot from the USB device and then say it does not see a bootable device. I would have to unplug the USB device, plug it back in then reboot my system for Unraid to boot. I ended up replacing my USB device since it was acting strange. My guess is it was on its way out the door. I haven't had an issue since I replaced the Unraid USB device. I'd say replace it every couple of years since USB read/write life expectancies are shorter than traditional HDDs/SSDs.