File transfer problem

[Ripstitch]

Hi, I've used vlc for years without any problems. Now when I download a video file to my desktop it plays as normal. If I copy it to exernal hd or flash drive it doesn't play . I get a orange line moving across the play progress bar . This has just started to happen.
I'm no techie so if you could keep it simple id be very grateful .
Thanks any ideas welcome ...