I had this exact problem the other night with an IDE drive.. It just froze up
during an IO intensive operation..  Machine immediately HUNG solid..  was about
to reboot when I noticed it came back to life for a breif second.  

The drive powered itself off for whatever sick reason.. but this caused the
update daemon to hit 400% CPU (according to PS.. yike)  and it just kept doing
that everytime you breathed on the drive.. If I stayed away from it it was
fine..  The annoying thing was that I could not unmount the drive.. at all.. 
eventually a cron job kicked in and locked it solid.. I was forced to power
down.. (ugh).

Drive is fine now..  but I'm pulling everything off it.. Either way.. it was
really annoying.. there was no way to forcibly unmount the drive..  So I was
stuck with an eventual disaster of some sort.

---
Tim Rightnour    -  root@garbled.futureone.com
http://www.zynetwc.com/~garbled/garbled.html