Too hot to handle...
IMHO, it is a cooling problem.
The design of the 721 and its sibling 522 (and presumably, the 625) suffers from poor design for heat dissipation. The longer it runs, the hotter the disk drive gets, and it is just about as far as it could be from the ventilation holes on the opposite side of the box. If you have it inside any type of enclosure, it just makes it worse.
gary, yours that sat for 1.5 hours and then worked again, probably was working because it cooled down enough to operate properly. It would have failed again.
My 522 hard drives live on the outside of the box, (power & IDE cables extended, note I was real careful about the "warranty void" sticker) so they are exposed directly to a roomful of conditioned air. Also before I did this, I made sure the units are NOT inside of anything but on top of a shelf. I found some old plastic cassette cases and put two of those underneath each box so there's about another half-inch of airflow available. Sitting outside the box, the drives are warm to the touch but not dangerously hot as when installed in the sealed box. Barring accidental damage because they are exposed, I expect them to last a whole lot longer.
Rewiring the fan so that it's on all the time might be another solution (or, if Dish would change the software to lower the threshold temp that activates the fan). I also seriously considered cutting a hole in the top of the case to attach another always on fan there as well.