You need to bounce over to the TiVo Upgrade forum and start reading. It's not hard, but way too much for me to rewrite here.
First thing I'd do is put back the original drive and get the box operating while you read how to do it and gather the necessary tools/software/etc.
Hitachi 43FDX10B 43" Digital TV, Motorola HD/Comcast Digital cable, Phillips DVD, Sharp VCR, Tivo Series II - 300 hr w/Lifetime Sub and HMO (with its own Comcast box),
Farallon NetLINE USB 10/100 Ethernet Adapter, Comcast Cable Internet, Sony HT-DDW740 Home Theater System, Comcast Digital cable, All operated with a Sony RM-AV3000
Tivo Series II - 60 hr (Stock) w/Lifetime Sub and HMO, Comcast Digital cable, Linksys WUSB11 Ethernet Adapter
I can no longer make backup images available. Requests will be ignored, sorry!