Unfortunately I don't have anything to add as a suggestion, but I just wanted to say that I've had this same problem. For me, I don't have a remote way into my box, so I have no way of killing whatever process may be doing this, but just recently I had this happen upon opening my email and it corrupted most of my email directory. I also had it happen while I was bunzip2ing my new kernel source, and the resulting tar file was corrupt. Both times I had to do a hard reboot (ala power switch) to get any results. One thing I've noticed, though, was that even though the hard light was on, there wasn't actually any sound of activity coming from the drive.
Just my pennies worth of info... Jeremy
