To take apart your TiVo, you need a T20, T15, and T10 Torx (star) drivers. Remove the screws on the back, slide the top of the case backwards, and your in. Remove the hard drive connectors and unscrew the hard drive bracket to remove the hard drive.
Once you are ready to install your new / modified hard drive, repeat the steps.
This video shows how to replace the hard drive in a TiVo Series 3; the process is basically the same for all TiVos.