Has your MP3 player got a button marked lock or hold. If so try switching this the other way as this can prevent data transfer. What make & model is your player?Someone may be able to help.
Just what are you asking? If you want to copy a cd to an mp3 player simply plugh the disk into windows media player then rip it. once rip simply add it to your sync list. It wont work if you simply copy and paste to your mp3 you HAVE to add it to your sync list.