If you had accidentally tried to read the Tivo drive in windows and such as assign a drive letter, but didn't format it, you can fix it so it can work again in the Tivo.
Use WinMFS, "Fix BootPage"
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!