I've tried them all at this stage -- don't think this motherboard has USB 2.0 specifically, nor any way to disable the USB 3.x ports.
Good point, but no. I just tried putting Ubuntu on a USB via Rufus and I get the initial splash screen where I can hit "try Ubuntu", but immediately blank afterwards. This is the same behaviour as unRAID, so I don't think it's unRAID specific.
Weirdly on that PC I can put Windows 11 on USB, boot, install and use the OS without issue.
.. This is really confusing.
The USB in question came out of the working unRAID server before I rebuilt, so yes. However I flashed it since during testing. Will give it a go on another system now.