I unhooked it this morning and took it to work to see if I could diagnose the problem.
Turns out it's the fan on the power supply. The fan has a speed sensor, and because the fan is clogged up, it's not turning fast enough, and it sets off the diagnostic condition.
Bitch is it's a small fan, like 1-1/2" square. I might be able to clean and lube it enough to spin faster, and maybe even fast enough to work long enough to pull the files off of it, but I suspect it may be time for a new fileserver.
Edit: cleaned and lubed the fan with some PTFE lubricant and it seems to be back working again. I'll know more when I hook it back up tonight.