@miikerman wrote:
As noted above, I’d try reformatting. Also note: some people have reported issues with above Class 4 speed microSD cards (unfortunately).
Are you sure you didn’t mean Class 10? Class 4 cards work like a dream. 
This post(although referring to phones) may shed some light on the issue and also be applicable to mp3 players. In fact, probably even more so (provided one agrees with the opinion).
