I have this problem. In my lumia 620 (8 gb internal memory) i cant store the offline music of my playlists in my 32 Gb external SD, there's no option in the settings and always send me error beacause i dont have enough space in my phone.
I set in my lumia that all the files (music, photos, videos etc etc) must be store in the external SD card, so i dont think that these was the problem.
How i can change the drive for store ???