Files won't play

I am having problems playing files after I copy them to my permanent drive. I have a 2TB hard drive permanently hooked to the wdtv. I can play files from my portable drive but when I copy them to the 2TB drive some of them won’t play anymore. It makes no sense because if I copy a music album some of the songs won’t play some will, but they all played from my portable drive. This happens with movie files as well. Unfortunately I lost some irreplaceable documentaries when I moved them from my portable drive. They are on the 2TB drive. The file extensions are all correct, but they won’t play anymore. Any ideas?

Scan your drive for errors, you might have a corrupted file system or bad sectors.