You can't move hard drives between TiVo's as the recordings are keyed to the TSN of the TiVo they were recorded on. You have to do a Clear and Delete Everything to marry a drive to a TiVo and that deletes all shows. I'm a little surprised that it actively deleted the shows (versus just not being able to play them back) but others have reported something like this with regards to new recordings which disappeared on a reboot when they did not do a Clear and Delete after imaging a new drive or installing a drive from another TiVo.
Unfortunately, I don't think you will be able to recover the shows.
Scott