RossMAN, for my first upgrade, I added an 80 gig Maxtor 5400 rpm drive to the original hard drive. That setup worked great for over a year. Then I decided to upgrade the upgrade. I used the 80 gig Maxtor and added a 120 gig Maxtor to it, no longer using the original drive. The upgrade itself was very easy to do. TiVo users have a great forum with lots of experienced people to help with any questions. Not sure about Replay however.