I tried moving the cache from my ssd to my hdd but the change has no effect. Spotify creates a folder on my hdd but does not use this folder (the size stays at a few Kbytes). The cache on my ssd keeps growing...
I also had this issue. I solved it on the Arch linux by using symbolic links provided by OS.
Lets say you want to move cache location from ~/.cache/spotify/ to /mnt/hdd/.cache/spotify.
1) Close spotify
2) Move all data from ~/.cache/spotify/ to /mnt/hdd/.cache/spotify
3) delete ~/.cache/spotify
4) Issue following command in terminal:ln -s /mnt/hdd/.cache/spotify ~/.cache/spotify
5) Test it by running Spotify