The 5xx series, like I said earlier, REQUIRE a SPECIFIC hard drive. The 501/08/10 can ONLY use 40, 80 or 120GB drives. I'm not sure about the 522. The 721 can't see beyond 137GB due to the limitation of the IDE controller(there is a Linux kernel patch to get around this, but I don't know if it would work). Drive sizes are generally limited by the hardware on a lot of DVRs. Most TiVo's can't use a HD larger than 137GB either.