I do think cloning the bad drive to a good drive with ddrescue is a good idea using several passes. Would boot the clone in the TiVo and do a KS 58 almost immediately. That should check/update the MFS and MFS OS which should take care of everything that was a problem in critical areas. You might have some bad spots in recordings but everything else should be good. Then do several force connections with TiVo servers. If all goes well, you should be able to copy that image to the 6TB drive. I recommend booting the copy in the TiVo before expanding it. Again, I recommend a KS 58 followed by a couple of connections to TiVo servers. If all goes well, then expand the drive.