You can add an eSATA drive to expand storage, but it's formatted using TiVo's internal file system. Also all recordings made after the drive is added are spread across both the internal drive and the external drive. They do this as a form of copy protection. They don't want you to be able to remove the drive and get the recordings off of it.
There are some NAS systems that support TiVoToGo and can automatically pull new recordings to the NAS as a backup. However TTG prevents transfer of protected shows, so this will only work for shows that are unprotected. (most cable companies protect at least the premium channels, and some protect all but the local channels)
Developer for VideoReDo