This is B*****bleep**. I've waisted so much time trying to find a solution to this problem. I appreciate people on here helping others out using third party sources, but I'm not paying Spotify a subscription to have to waist my own time trawling the internet trying to find solutions that are then going to take me even more time and effort to implement.
Why can't spotify just release a new version with the ability to link files to external sd?
There are a number of threads relating to this topic, and I'm going to try and encourage users to follow just one for updates rather than multiple different ones. It seems that this one has become the one with the most replies, so it might be worthwhile subscribing to that thread for any further updates.