S5 is only reading 8GB of a microSD with 40GB data on it?

[AC Question]

Standard S5, running Android 5.0. Put 40GB of .mp3s on a brand new Samsung 128GB micro SD.

Once the card is in the phone, it only "sees" around 8GB of the files. However if I plug the phone into my PC, I can see all 40GB.

Tried reformatting and reloading the files while it is mounted in the phone, but same result. Any clues?