Can't move video files onto external hard drive?

I have about 250GB of video on my C drive which is obviously taking up a lot of room on my 500GB laptop, so I've been trying to move my video files (movies and tv shows that I've downloaded) onto my external hard drive. But half of the video transfer with really, really bad quality or when I try to open it up with windows media player, Windows Media Player gets a popup saying "Windows Media Player encountered a problem while playing the file". Please help! :)