Not a good sign. Had Disk Utility do a fsck, which it complained about, so I reboot to recovery mode…and with nothing running, this little gem appears.
Guess I’ll be creating a boot thumb drive then attempting a full wipe/rebuild that way instead.
Update: or not. It simply refuses to do any low-level process on the drive. Appears completely pooched. Time to order a new drive – and some tools to open the case.