Comentário oficial
You've already moved the database to an external drive, simply start rekordbox without the external connected, it won't be found, then it will warn you and ask if you want to load from the default location, say yes. It will create a new, blank, collection. After that, quit rekordbox, connect the external drive, boot rekordbox - it will load the internal database, but you can now switch between the internal and external by going into the preferences.