How can I upgrade my TiVo?

Sam wants to know how to increase the hard drive space on his TiVo. He's using the TiVo Premiere and it's running out of space. Leo says that AVS Forum is an ideal site for tips on doing that. There's also Weaknees.com, which sells TiVos with upgraded drives and sells the parts for the upgrade which are "blessed" to run on TiVos.