InstantCake is the fastest, easiest and safest way to repair, replace or upgrade your TiVo hard drive without purchasing a pre-configured kit or installation service. Boot the custom CD in a standard PC and create a replacement drive for your TiVo DVR.
Currently 1 TB HD is the biggest HD you can upgrade to with this software.
I unplugged my primary HD and plugged the cables into the replacement HD.
Booted CD. My settings were HDA for the replacement HD and HDC for the cd drive. Note that you can press shift+up to see what assignment your drives have.