As someone that has now gone through the ringer with tech support from Nokia and SanDisk/Patriot/Kingston about an issue with Nokia Music - I am going to start with the questions there...
Have you downloaded or used Nokia Music on the device? Is your card a UHS-1 card or just a normal Class 6 or Class 10 card? And finally, what program did you drop the music on to the card with?
What I have figured out is the following:
1) Downloading/Using Nokia Music screwed up my card - not only for music, but also for photos/video. The problem begins with the phone freezing when taking an image. It then has the SD Card Error message pop up immediately on reboot.
2) When transferring music, you must go into storage, remove the SD card and insert it into the computer - UNLESS you are using Windows Media Player in Win 7 or Windows Phone App for desktop in Win 8...
3) You cannot sync to more than one computer or the card gets really confused - don't try and sync to your desktop and then try and sync that to a laptop later. It will dupe the tracks an create all sorts of issues with album art.
4) Once there is an error on the card, simply repairing it in the PC will not fix the issue in the phone. It will temporarily, but not long term.
5) The type of card is also very important - make sure that the card itself isn't a defective card out of the box. Write errors are easy when the card isn't working right to start with.
I'm sure I'll have more soon as we continue to play with more scenarios on this one, but for now, that's what we've figured out.