I'm seeing the same type of thing trying to move files to and from my firewire drives. I've got about 40 gigs of content I'd like to rearrange, but they keep croaking with -36 errors and dismounting. One of them was failing to mount, and Disk Utility reported that the underlying task failed when I tried to Repair it. So I started copies from the other FW disk to the internal, and it spontaneiously unmounted with a -36.
Then drive # 1 magically reappeared. The one that DiskUtility couldn't repair. Seems to be intact....
But something's not right.
17" Duo Core iMac, 1.5 gig RAM.