@Robder > The backup feature doesn't "move" the music, it creates a copy. The easiest thing would be to simply copy your music folder from the internal drive to the external drive, then select all your songs in the rekordbox collection and use the relocate tool to point them to the new location, and finally, move your database to the external drive as well so you can move the music around!
What you've done is destroy your collection, the best course of action is to locate your backup database (provided you haven't closed/opened rekordbox more than 3 times) and restore that.
Are you on a Mac or PC?