If you had the current Tivo software (11.0k and up), it will support up to 2TB drive. Just use WinMFS to backup, restore, expand and supersize.
Original S1 (Recycle Heaven)
Series 2 (PLS, sold)
TiVo HD 1TB (157 HD Hrs, PLS, "652")
Juliet Basic (4XL, PLS)
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!