Failing Tivo Premier - Moving Hard Drive - Recovering shows

I have two Tivo Premiers, and one is failing. The bad one still boots and partly functions. Live TV generally looks OK, but newly recorded shows have pixelations/dropouts when played back. An hour long show recording will only be about 52 minutes long. Shows recorded before it had problems playback badly. The video will pause for 5-10 seconds every 30 seconds or so. The menus work but are slow, sometimes very slow. It tends to work worse the longer after a reboot. When first booted, it almost seems normal for a bit, but it's short lived.

I already purchased a new Bolt, and tried transferring the recordings to it, but it (essentially) did not work. One program finally transferred over about 24 hours, and it played back with hesitations/pixelations. (Perhaps it was a recently recorded show. I'm not sure.) On whim, I connect up a USB network adapter and rebooted, but it made no difference.

I've read a few threads here, so I understand I can not move the drive from the failing Premier into the working Premier and watch the shows already recorded (or transfer them off to my new Bolt).

Where I need help...
- What is likely the source of the failure? HD? Main board? Power supply?
- Is there a reasonable path where I get the shows off. For example, if the problem is the power supply, I could swap it. Or do I just walk away? (I'd certainly like to have the shows, but it's nothing I can't live without.)

The HD is a 3 year old WD 2TB green drive.

Thanks.