Why does the note 5 file manager recognize USB memory while MP3 players do not.

I've installed a 64 GB Sandisk dual USB drive in the USB port on my note 5 and transferred 4000 tracks of music to it in a folder named (oddly enough) Music. I've tried 2 different file manager apps and they both can work with these files. However, the 3 MP3 player apps I've tried do not look beyond internal memory for input and apparently do not have the capability to be told where to get tracks from.

One other item of note is both the file manager programs will play a Single tune in a bare minimum MP3 app.