Just my .02
I have had more than a few problems with heat-dissipation on external hard-drives. I spent two weeks thinking I had lost a bundle of data, tried all different things to recover the stuff. Turned out the USB connection for the external drive had failed. Once I connected IDE internally the drive worked. Immediately backed up everything to DVD/CD. Not one read error since.
External drives are handy, but not what I would call reliable.
Investigate very carefully.
I use DVD's or CD's for reliable backups (of course labeling would help
) . I have taken all of the covers off my external hard drives or connected the disks inside the tower itself. We do the same thing at work. When you do that, you need to be very careful about grounding.
Most of these external caddies get extremely hot when on for long periods of time.
Like I said, my .02