The usual is the player is paused during the attempted transfer. It's a common pilot error issue.
Your post contains so little information that this is all I can offer.
all of a sudden when i try to copy my music from my mp3play lists on my pc to my mp3 player it is coming up with a "error disc is write protected" i've been copying to my mp3 player for months so why is it doing this now ? i'm puzzeled?
any help please