This post is loosely related to PC's in that it uses a PC hard drive. If you happen to have a Scientific Atlanta Explorer 8300 HD cable box, and a cable company who didn't disable your E-SATA port then you can try this.
1 external (and powered) E-SATA capable drive (1.5Gb/s will do)
1 esata cable
1. Disconnect the power from the cable box
2. Connect the drive to the cable box via the e-sata port, plug the drive power cable into the outlet and turn it on, and connect the power to the cable box.
3. After booting up, the box will give you a prompt asking if you want to format the drive. Say yes, and wait until the drive calms down so you know it is done formatting. Check your DVR menu, and if the % of spaced used has not dropped, you will need to disconnect the power from the cable box again, and then it should work properly and give you a message that it is using the drive (keep doing this last step until the % of recording space used drops).