The whole Idea of the SD is was to help lighten the load on your phones internal memory storage, use of the SD cards on Cellphones had data back all the way to early flip phone, Black Berries & others..
I understand what the idea is behind having an SD card on my tablet. What I'm saying is that I am unable to download apps, pictures, music or videos to the SD card. Therefore I am constantly receiving error messages telling my that I do not have the space available to download anything. I was able to put music on the SD card by removing it and then putting it into a card reader then downloading the music directly to the card. So far, that's all I have been able to do.
This may of been ask before but did you try formatting the card say like the first time you turned on the phone or if you purchased a bigger SD card in your settings you can Format the card and it should help with these issue as your seeing..
What i suggested you can use as a future option, but since what is working for ya as you mentioned you might want to stay with it as it's a way to enjoy your music.
Have a Good Afternoon.!