You can't (at least no one has figured out how to) replace the
hard drive that comes with your Tivo. HOWEVER, each Tivo comes
set up for two drives. There is a second mounting platform, a second
IDE connector, and a second power connector in the case. All
you need to do is add the second drive and connect everything up.
Before you add the second drive, you need to write a few small
bits of info to it ("bless" it in the vernacular of Tivo hackers).
You connect the drive to you standard desktop, then run a
special Linux boot floppy (widely available). It takes all of
about 30 seconds to run. Once the hard drive is "blessed" you can
install it in your Tivo, and the next time you boot the Tivo up,
it'll see the new drive and you'll have bazillions of hours of
record time. (Well, 91 hours to be exact, if you started with
a 15 hour Tivo and added a 60Gb drive, which is what I did).
DETAILED instructions are available in the Tivo forum at
www.avsforum.com.
Kwad