I was having trouble, too, trying to get my TIVO HD to enter the Kickstart mode. I tried 10 times, without success, before finding the key: patience. The instructions say to press the pause button for a few moments before the red and yellow lights come on. Turns out in my case a "few moments" was 90 seconds or so. Finally the red and yellow lights came on and I pressed Kickstart code 54 for the hard drive test. Again, I thought I failed because the TIVO just kept going with powering up. Again, the key was patience. I was about ready to give up when up pops the hard drive test screen. Success! My advice: this takes longer than you think it should.

Older units can boot more slowly.

On my TiVo Premiere XL I find that if I hit the Pause button when the yellow light flashes then enter the KS code very soon after, I could get into the KS mode every time. In the case of the XL, entering the code is successful if the green and yellow lights flash alternately. I've also found that if you see all that and you still can't get the codes to work, that the OS on the disk may be trashed.

When my XL got in this mode, I pulled the disk and used DvrBARS try and read the disk and DvrBARS encountered the error: MFS volume header not found. The response from the author of DvrBARS in the TiVo Community forum was that I was pretty much out of luck recovering the disk.

Then I ran full diags on the disk and it was fine. So I backed up and restored a new image and the box is back up and running. Time will tell if it fails again.