I don't know about that one but I just got a Cavalry 1 TB from newegg.com for $194.99 w/ free shipping. I've had it for 2 days now and it is working fine with my HR21-700.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822101082
I have two FreeAgent Pro 750's that work fine with my HR20's but I've read on this site that the FreeAgent Pro doesn't work with the HR-21.
Edit: If the Best Buy hard drive is a "My Book" be aware that those have been reported in the eSATA thread as not working with DVR's. See the thread here (look somewhere around Nov / Dec 2007 for the reported problems):
http://www.dbstalk.com/showthread.php?t=66201
http://www.newegg.com/Product/Product.aspx?Item=N82E16822101082
I have two FreeAgent Pro 750's that work fine with my HR20's but I've read on this site that the FreeAgent Pro doesn't work with the HR-21.
Edit: If the Best Buy hard drive is a "My Book" be aware that those have been reported in the eSATA thread as not working with DVR's. See the thread here (look somewhere around Nov / Dec 2007 for the reported problems):
http://www.dbstalk.com/showthread.php?t=66201