I was surprised to find out in the last few days that you can backup your TiVo internal hard drive and subsequently also upgrade/replace it.
We have two TiVo’s and unfortunately the WD DVR expansion units we had on both have long since died unfortunately, and now I know we can upgrade the internal drive. I pulled one of our units apart and hooked up the drive to a Windows XP machine and created a truncated backup. Just incase, below is a screen grab from the Windows application I used.
After you select the drive, you can run a backup of the drive. Which I did quite promptly.