I have to give props to “Hollyman” blog. I just expanded my DirecTV HD-DVR to 150+ hours of HD storage. I got sick of the constant inability to record HD football because my DVR was running out of space.
The solution? Pay $200+ for marginally more space (HR23 model) or take my HR21 and expand it with an unsupported feature – External hard drive.
I did the latter. And for $150, I expanded the drive from ~300GB to 1 TB (1000 GB)! It was literally plug and play.
For the basics, check out the post cited above from Hollyman. In it he’s expanding an HR20 to 1 TB. That’s incrementally more complicate (has to do with the drive transfer rate and changing the drive from 3GBps to 1.5Gbps. I don’t recommend that. If you have an HR20, it’ll take a little more electronics tinkering (using jumpers to “downshift” the drive).
If you have an HR21 (like me) just skip the step about down-shifting the speed of the drive and go for it! I followed his steps exactly, down to the exact equipment, and it works perfectly. I have a bunch of HD stuff recorded in the first couple days and its still at 97% empty! This rocks!
Wow, I’m a tool…but a tool with a HUGE DVR! 
