You're generally better off "building" your own.
Buy a harddrive. Buy an enclosure. Screw the drive into the enclosure. Done.
As for drives, buy the cheapest one you can find with the best warranty. Generally you can get 3 year warranty and if the drive dies in that 3 years, youget a replacement (although you lose the data - BACK IT UP!).
If you need recommendations on the enclosures, ask -- I have the Antec Veris, but I haven't used it yet; it's compatable with the DirecTV HR21 though, and was only $20, which is why I bought it.