There are reports that on the latest Android devices you can now store your music on an SD card and the Google Play Music (GPM) app can play them.
This is incorrect. GPM can store cached music from its own service to an sdcard. That way you can play them offline without losing valuable memory on your device. It is also Google’s way to lock in content and services; maybe due to industry legalities?
Another music player app, Poweramp, can play music from an SD card.
Anyway, if the above is incorrect, please respond with directions on how to copy music from a device onto an Android phone and allow Google Play Music to access it.
I tried all this on a very recent tablet running KitKat.