Comentário oficial
What you want to do is connect the USB drive back to rekordbox, then right-click on the drive and select "Update Collection" and that data and changes from the USB drive will update the tracks in rekordbox. If you made changes to the same track on both the USB drive and in rekordbox, the update will overwrite the changes with the information from the USB drive. Any track without changes won't be affected, and likewise, any track not on the USB drive won't be affected.