The Tivo expander drives are the only supported external drives that are Plug-N-Play. Others you have to remove the internal Tivo drive and hook up with the external and use WinMFS's MFSAdd.
Keep in mind that if either drive fails, you will lose recordings that were stored since the addition of the external.
Series 2 (Retired, sold)
TiVo HD 1TB (157 HD Hrs, PLS, "S3HD")
Roamio, Oh Roamio!
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!