That law basically says a company cannot void the warranty of unrelated parts. Now there are two things about the TiVo that make it different from the reason that law exists (i.e., a car). First, a TiVo is a box that's "no user serviceable parts inside". Which means there's no reason to go rooting around inside because the user is not expected to be able to replace parts. For any reason, and these days, liability because of stuff like the open-cage power supply, sharp edges, etc. The user is free to open their product (it's theirs), but the manufacturer is no longer obligated because the product was never designed for user servicing. Second, the law states they can't void the warranty of unrelated parts. So if you change your radio, the manufacturer cannot void the warranty of the engine, unless they could show a direct relation between the radio and the engine, and that modifying one can destroy the other. In this case, the user mucked with the bad part, and TiVo no longer warrants the hard drive because the user directly mucked with it (assuming they were meant to). Next, the hard drive is pretty critical to the entire operation and it won't be as hard for TiVo to show that mucking with the hard drive could cause potential damage to other components like say, the motherboard, the power supply, etc. Take a slightly different system, like say a desktop PC, and it's a lot clearer the user was meant to wander inside because the user manual states how to do stuff like add memory, add hard drives etc. In which case the case was intended to come off to allow user serviceability because it's documented right there. In TiVo's case, nothing was supposed to be replaced.