Not only can you add an external drive, it is easy. Just go to Amazon and order a drive and an enclosure. When they come in, stick the drive in the enclosure, power down the Directv DRV, plug in the eSata connector and the enclosure power, turn on the DRV, and it comes up like a new DVR, blank with MORE room. It is like new and you have to set everything up ( favorites, recordings, … ). Any recordings you had are not available ! You can access them any time by powering down, disconnecting the eSata cable, and powering up but it IS either / or and you really don’t want to be swapping back and forth with recordings coming in on whichever one you have up at time. You need a clean swap and stay with the external. It really does work to go back though !
The Genie will take a 3T drive, which is what I did. I went with the Western Digital WD30EURS. It runs silent and completely cool. I was amazed at this “green” drive made to run full time would be so quite and cool. It is an AV model made just for video and DVRs and its 1T version is what is in the Genie. I really think that an enclosure without a fan would do. Surely left open. I went with a really good quite one with a fan that can be switched off. It is the Rosewill RX-358 V2. It allows the 3T drives whereas some of the others say they only do up to 2T. It is quite, though not silent like the drive. I did button it up and I run the fan but I think it could be placed properly without a fanned enclosure.
After it filled up to 89% free it had 120 hours of HD recorded I concluded that it was indeed using the full 3T ! I should get 1000 hours of HD.
If you want to add an external hard drive to a HR24 or other non-genie DRV it appears that you can only go up to a 2T. Since it is going up from 500 GB, that is 4 times the storage. I would go with the Western Digital WD20EURS instead of the Western Digital WD30EURS which is $92 instead of $145 at Amazon.