OK, so I decided to update from my series 2 to the new THR-22 and after a couple of weeks of use realized that the 500GB drive would not cut it and I needed to upgrade it.
So I though I would use a video surveillance HD I had; the Seagate ST2000VX000.
This upgrade resulted in the pixelation people have reported and slow menu response too. (Even after a couple of weeks of use)
So then I tried the Seagate Pipeline HD (ST2000VM002), this had less of a problem but still did not work flawlessly like the original 500GB pipeline drive.
So I did a bit of research and decided to try the Hitachi Deskstar (H3D20006472S) as it was a highly rated 7200 rpm drive, used older, none-AF technology and a 6Gb/s interface. (Although our Tivo's can only use the 3Gb/s mode, I knew the drive itself have to be fast to be able to feed the 6Gb interface)
This drive worked flawlessly from the moment I installed it. No waiting for it to settle down and menu response was fast too. The drive is a available for around $100.
I also ran some test on the drives and here are the results using tests by Spinrite:
Burst Transfer Rate
Hitachi H3D20006472S = 88,896,060
Seagate ST2000VX000 = 64,984,160
Seagate ST2000VM002 = 51,225,989
Sustained Transfer rate
Hitachi H3D20006472S = 124,624,287
Seagate ST2000VX000 = 117,498,617
Seagate ST2000VM002 = 109,157,670
Random Sector Access
Hitachi H3D20006472S = 12.263
Seagate ST2000VX000 = 9.189
Seagate ST2000VM002 = 13.703
I tried testing the original 500GB drive but for some reason Sprinrite would not recognize it. (I wonder if it is modified in some way?)