1. You do not want to do that. You should put the original hard drive in storage just in case something happens to your upgraded hard drive.
When that something does happen (and it will eventually), you'll have the original as a backup.
2. IIRC, once you upgrade the internal hard drive, the Tivo will no longer accept connecting an external.
The Man Prayer: I'm a man ...... I can change ...... If I have to ...... I guess.