My wife recently ordered a 160GB add-on drive kit from weaKnees for our Hughes HDVR2 DirecTiVo and we installed it last Thursday night. It was a quick and easy install, though the only way we could tell the drive had been properly recognized was to wait until we had recorded enough additional programs to go beyond the 35 hour limit of the original 40 GB drive. Our DirecTiVo now can now store 155 hours of recordings, since the 160 GB drive can hold 120 hours.
It would be nice if the TiVo told you how much drive space was left. The only way to get it to display that info is to enable Backdoors mode. Unfortunately, enabling Backdoors mode on a series 2 TiVo with OS version 3.1 or later is really time consuming. If you have a DirecTiVo that is not hacked to enable the USB ports (which is also a massive pain), you have to remove the master drive from the TiVo as part of enabling Backdoors mode.