Start here
http://www.tivocommunity.com/tivo-vb...d.php?t=141538
I would replace the drive then store the original drive as a backup. After the second drive is "married" you can not go back without loosing recordings. A Failure of one of the two drives you will have to restore from the backup if you have one.
When you get the drive make sure it is a EIDE (PATA) drive the TiVo does not have SATA connections.
If you still plan on adding a second drive check out the sponsors of this forum you will need a mounting kit.
As for other "hacks" there are very few that do not require hard ware modifications.
NOTE: Removing the cover may void the warranty.