You wrote that the tracks are not in folders, but when you click on a folder, it does not show me the tracks inside… Hmmm, ok...
I suppose that your external drive is hooked up on to your laptop or PC, on which Rekordbox is running. So far so good.
When you want to use the tracks on your external drive, you need to select them all in Explorer/Finder and drag-and-drop them in to the main collection folder of Rekordbox. This is no copy-paste action from external drive to your PC/Laptop and Rekordbox, but a file alocation action. No files are copied, Rekord box is only told where to find all your tracks now, and shows them in the Rekordbox main collection.
In Rekordbox drag-and-drop a track from the main collection in to one of the players/decks, and you will be able to play the song.
When you want to make music collections, you can add folders in Rekordbox, then drag-and-drop tracks from the main Rekordbox collection in to the separate folder. Have a look at some YouTube tutorials about Rekordbox and track management. It's very helpfull.