Much to my surprise, Directv is finally offering up advice for expanding the hard drive capacity or recording capacity of their DVR units. This is something that I struggled with on my own last year after trying out numerous external eSATA hard drives. I even broke the seal in the back of my HR21 (and the warranty) in a desperate effort to make it work. Long story there, I got it to work utilizing a Maxtor L01F100 1 TB hard drive coupled with a Veris MX-1 hard drive enclosure. This worked great, for about 2 or 3 months, until the hard drive started freezing and I had to constantly reboot the box. Not happy, I finally gave up and resorted to the much smaller 250 GB standard internal drive. Man I missed the days of being able to record multiple, complete series of TV shows in HD (like all my Formula One programming, Big Love, etc).
Much to my surprise and renewed interest, my wife forwarded me a Facebook post from Directv's Facebook page that lead to a Tips & Tricks article on the Directv site: Increase Recording Capacity on DVR or HD DVR. It's very basic but there's other critical information you'll need in order to make this work. Here's the article highlights with my own tips:
This is a nice, easy option for most consumers, assuming that it works. The other option would be to send out your DVR to a professional and have them install a new, bigger hard drive. This is more expensive but also more likely to work (not to mention the work is probably guaranteed) and also means that you don't have to worry about the sound of the external drive spinning or needing additional space for the external drive.
Have fun with the extra room. Now you can record the entire Sponge Bob Square Pants series along with all your favorite HD programs, worry free!