Without looking at the content of your drive before you attempt to add it to iTunes, and not knowing what settings are in place, it is hard to be certain what the issue might be. For example there could be multiple tracks in the source folder, which is why adding them makes them show up in iTunes. Adding an XML playlist at the time of the import might also cause problems though I've never tested for this. However there should be no need to add media to a new library. If you have properly copied over your content from a previous computer then you should be bringing over the complete working library with all of its ratings, playlists, play counts, etc. in place.
In general you would copy the entire iTunes folder from the user's music folder on the old computer into user's music folder and it will just work. For those who have split out the media to a new location there are steps in Make a split library portable which may be needed to make sure that the library is still able to see the media after the transfer.
tt2