Whilst in the spotify application go to the settings. Almost at the bottom of the settings menu should be a section called ''Storage'. Enter this and select your SD card, the app should then tell you it will transfer data to your new storage location.
Please use the search feature before posting a problem/bug/idea. What's a Spotify Rock Star, and how do I become one? Using Spotify Since March 10th 2009 Twitter: @mechanimal82