They can and do. "Please note that removing the cover of the DVR for any reason voids the warranty." http://www.tivo.com/buytivo/dvrlimitedwarranty.html This is not a laptop or desktop computer where a consumer could reasonably expect to replace or upgrade parts without voiding the warranty. Yes, plenty of people can and do upgrade their hard drives. I have no idea of whether TiVo is strict about this clause or not, but they certainly could be.