I suggest you download and burn yourself a copy of the Ultimate Boot CD.
It has hard drive tests from each of the major manufacturers (among other things).
Basically, once you burn the boot CD, configure your PC BIOS so that it will boot from the CD/DVD drive then shut it down.
Connect the Tivo hard drive to the PC. I *think* a USB adapter will work, but a direct SATA connection will be faster.
Disconnect any hard drive that you don't need to test (not really necessary, I just do that as a rule to avoid mishap).
Boot the PC using the Ultimate Boot CD and find the diagnostic software (most likely Western Digital). Perform the short test first. If it passes that, then run the extended test.