I'd like to start off by welcoming you to the forums and saying that I'm really sorry to hear that you're experiencing some issues with your TiVo® recordings.
You've hit the nail on the head when you say that the hard drive is failing. Tests that I've done from this end have confirmed that.
At this point I'd like to arrange for a technician to call out and investigate further for you. I've sent you a private message to arrange this, please look to the top right of this page where you'll see a purple envelope icon, click on this and you'll see my message.