I'm a little late in responding, but at this point I would recommend avoiding that model of Seagate drive at all cost for that configuration. There is a thread on DBSTALK.com (here) specifically about that drive. I purchased one with the MX-1 enclosure and it does not work correctly.
There are also several reviews on Newegg that indicate a potential QA issue with that model. Those can be found here.
Wow, thanks for the info!
I'm glad I went with the cheapest stuff I read would work with the HR21...I ordered a WD 1TB drive (WD10EACS) and a Rosewill (RX353-S) enclosure with Esata cable for $180. Should get them tomorrow and I'll report my results...