It looked to me like the CPU had died rather than anything else, so I brought up another server (we do tend to have them lying about the place), swapped the disks in from the old box -- yup, everything fine, so let's take a backup before doing anything else.
We have, the figures suggest, somewhere around 85Gb of stuff to back up on that server, split across two disks.
The server which will shortly be back on the network with the MP3s on, while entirely happy to run a nice modern Linux kernel, only supports USB1.1.
This could take a while....