It may not be relevant, but I would suggest looking at http://www.tivocommunity.com/tivo-vb/showthread.php?t=350510
The first message on the thread shows that external enclosures and drives that have been verified to work with the Tivo S3. I would suspect any of these would work fine with the HR20 or HR21.
I am currently using a Hitachi 1TB Deskstar drive in an Antec MX-1 case on my Tivo S3. I am also using a FAP 750GB with my HR20-700.
Neither of these have had any problems since I connected them over six months ago.