It is easy to upgrade the hard drive inside the TiVo HD itself, but as Trevor said you will void the warranty. WinMFS is a simple and easy way to upgrade to any SATA drive of your choice. I replaced my TiVo HD drive with a 750 just over a year ago and it's been running great with ton's of storage space. And I still have the original drive if I need.

Edit: Doh! Should have read Trevor's link before posting WinMFS which is mentioned in his link. Of course it's way down on the post before he mentions it.