Well I've looked into it a little further - I've got a USB Notebook harddrive caddy, so I dismantled the DMS, slapped the drive into the caddy and hooked it up to my PC - Blam, instant MP3s.
So that prove the drive's still working fine - I'll check the cable in the keg tomorrow morning, hopefully that's the source of the problem.
Trust me to fix it just as I'm about to sell the car!