Do I under stand that after replacing the HD the first Power-Up, Almost There message you do not have any video even after a power off restart not even the Power-Up?
If this is true and you get the normal TiVo beeps and can play shows (Audio Only) with either HD. I do not think the problem is with the HD or even GSOD. The TiVo seems to be working there must be a problem with the Video circuit probably the video output amp if you've tried both Video outs and the RF out it may be time for professional service.