I have a S3 HD unit that I started using in Jan. 2008. I have had no problems until this week. A few days ago it started rebooting continuously and will not repond to any commands either from the remote or the front of the unit. The sequence NOW goes like this: Starts powering up, wait a little longer, Intro music then THX and TIVO symbol come up then TIVO Central screen, but all are double images and vibrating rapidly. Then after 15-20 minutes the sequence starts all over. I can't complete any of the diagnostics because the unit does not respond to the remote (which has new batteries and the light on the remote and the unit come on when any button is pressed) or buttons on unit itself. I started the suggested procedure of removing the cable cards, but after the unit powered back up there was no change. So......I'm not a computer genius but I do have some knowledge and this has baffled me. I think I have read every post on this site and couldn't find the exact symptoms or a test that I could run because it will not respond once it gets booted up. I did try the unplug, plug-in sequence many times and that made no change either. So what I need is some HELP !!!!!! If you think it may be the hard drive can I replace it with a larger one even though the unit is not operating correctly now?