External HDD too small to fit files on them.?

I have a 1TB HDD and I store all my movies and music and photos on it, I am trying to move a movie to my hard drive and it's only 5gb and the comp keeps telling me it's too big for the destination. I thought maybe it's because the movie was a .mkv format so I converted it into a .wmv and I still have the same proble help!