Can the hard drive in an Apple TV be upgraded? How?
Solution 1:
Yes, you can. Good thing you don't care about your warranty though. Engadget published an article about it with instructions some time ago.
Solution 2:
The iFixit folks have a guide, as well.
Solution 3:
Be warned that, once you've replaced the HD, you won't be able to update the Apple TV software through the usual means. So you'll have to either remove the HD every time there's a new version (and go through the rather elaborate procedure to prepare and reinstall it), or you can get used to dismissing the update warning every time you want to watch a movie.