The drive content is not tied to the motherboard. If the drives are *both* good, then they should work in another *same model* TiVo. However, the lifetime subscription is tied to your motherboard.
My first thought is that you probably have a drive issue. Although, I'm not sure in your case. In a dual drive setup, if one drive fails you are down. Most TiVo units I've seen fail have been hard drive issues, or the internal fan, and then the power supply (these are the moving parts). When you plug it in can you hear the fan/drives spinning up? If so then it is probably not the power supply.
If it is the drives, you have lost your recordings- but you can replace them with a new drive and download a software image at
http://tivo.upgrade-instructions.com/index.phpYou select your model of TiVo and then download the image (cost is $20 for the software). You then burn the downloaded iso image to a CD and boot a PC to the CDROM. Have your *new* TiVo hard drive connected as secondary master (/dev/hdc) and it will write the new image to the drive. There are more instructions provided on the site.
This may not be your issue, but I suspect there is a good chance it is. Maybe you don't want to buy a new 200 or 300 GB drive if you are not sure, but if you have a spare 40 or 80 GB drive around, you could test it and possibly get your TiVo running again at least.
My sincere condolences on your loss.
