You booted to WinXP, didn't you?
If you booted to Windows XP with your Tivo drive attatched (or you were screwing around with the Disk Manager in Win2k) your boot sector is probably trashed. Download MakeTivoBootable [.zip, compiled executable] to fix it.

(Copyright (and all that jazz) by Daniel Prantz (dmprantz on tivo-community.com), and with absolutely no warranty, etc etc.)

Tivo Partition Tools
The rest of the Tivo Partition Tools (MakeTiVoBootable is included) can be found here [tar/gzip], also by Daniel Prantz. This package features such hits as:

• AddFreeSpace
• ExtendedBlessing
• AddMediaRegion
• repart
• MakeTiVoBootable