I tried a 1TB FreeAgent Pro on my HR21-200 and it did NOT work. I also tried it on a PC (eSATA), with terrible results (REALLY slow!) In the end, I bought Antec MX-1 cases for the HR21 and the PC, and am very happy.
Perhaps I'm mistaken, but I seem to recall Directv sending me another form that I had to return to get the credit (signing up wasn't enough trouble?) Watch what they send you (just in case).