[MovedGoalPosts]

After poking around, a lot, and with a brainwave in the very small hours of this morning, I wondered if I'd somehow unseated something critical as I'd been messing around before the total failure trying to work out why a hard disk wasn't been seen. Somehow I'd disturbed the CPU seating, maybe just a pin or two wasn't making contact - the inside of the case was black with dirt so who knows what debris was in the way. Stripped everything out, including fully reseating the CPU with new thermal compound and the server has rebooted. Phew.

Still don't know why it can't see some of the hard disks, especially the USB stuff we used for backups. Still at least I have a working server again, and can at least run some sort of backup over the network so we aren't in quite the same level of vulnerability.