The only time I've ever had this problem is if I had a video and it was all tagged, then I deleted the video from OUTSIDE of iTunes, replaced it with a new video (such as a higher-quality encode) and then pointed iTunes to the new file. The metadata stays until I remove the video from iTunes.
However, if I add a video to iTunes and tag it, or if I use Metax, the metadata stays with the file, I never have to retag.
So if you used Metax or tagged the video in iTunes, you shouldnt' have this problem if we're talking about mp4 videos. Try one for test purposes. Delete it from iTunes, but not from your drive. Then re-import and see what happens. Is the metadata there?
Regardless, you can do this without losing metadata. Just right-click on any or all of the shows you want to move, and select "Consolidate" and choose a folder on the external drive. They will be copied over, leaving you free to delete them from their original location. Because the files never left the iTunes library, any metadata, whether in the files themselves or stored in the iTunes' library (like they are for WAV files), should still be there.
