Hi guys. I had a similar problem. Ive just bought my new Xperia z3. I had iPhone previously. On my iPhone i.had no problems with storage as I had 64gb. When I wanted to store all of my 18 gb of Spotify music on my Xperia I had a problem with the fact that no music from Spotify can be stored on sd card. So I found a solution. As you know android works strongly with Google. As Google realised an app called Google play which is a rival for Spotify and other music apps they made sure that only theirs music can be stored on sd. So the solution is resign from Spotify and buy Google play subscription which will allow you to store music from Google play ( which worked identically to Spotify) on your sd card.
That was weird. On my Z3 Compact it worked with reinstalling after the memory card was inserted. Now it shows on the PC and FileComander that there is nothing on the internal memory but that everything is on the SD card. What I do not understand is why it anyhow shows that inside the "storage" in the settings that it is still on internal memory.