I had this problem. It could be that the files were corrupted because you removed the card without properly unmounting it at one point or they were just on the card for too long - SD cards unlike regular hard drives and optical discs are not ideal long term storage solutions. What I had to do was simple - format the card and basically lose all the files you weren't able to copy. Or, you can keep it as is and keep trying over and over but chances are it will always get stuck on the same file or files.