It must be an HDD with its own power supply and a USB 2.0 port and SINGLE drive, none of those fat encasement that TWO HDD's for full capacity. The easiest would be to get the Western Digital My Book AV DVR Expander (1TB) with no guess work and is guaranteed to work with Dish, TiVo, DirecTV and other cable DVR's. Otherwise just about any external HDD meeting the above requirements could work for you.