The last paragraph here
explains something of what might be going on.
I get the feeling a new hard drive may be in your future.
While you're cleaing the rust off of your Torx drivers, you can look at this to see what you should have done
and then look at this
and try KS 0, 51, 52, and 56 in I have no idea which order, but at least the results might give clues.
When you do finally wind up taking the cover off (if it comes to that), hook the drive up to a computer and run WinMFS, select the drive, click on mfsinfo, see if it's set to boot from 2, 3, 4 or 5, 6, 7.
Then use the fix bootpage (or whatever it's called) to change that.
Option 1 will set it to boot from 234 and option 2 will set it to boot from 567, so choose whatever changes it from whatever it's set at.
Then click on mfsinfo again to confirm that it has changed.
Turn off the computer, put the drive back in the TiVo and see what happens.
Probably couldn't hurt to run the manufacturer's diagnostics on the drive while it's in the computer, either.
Good luck, let us know how it goes.