Error when copying files to iTunes

I recently ripped a few TV show seasons on to my Macbook but had to convert them because they weren't compatible with iTunes. The conversion went fine, all the files were perfect and worked when opened in Quicktime. I have iTunes running on an external hard drive. When I tried copying the first season of the show, iTunes returned with an error saying "Attempting to copy to "My iTunes Library" (my external hard drive) failed. The disk could not be read from or written." Then I tried copying the second and third seasons and they both worked, and are now in my iTunes library. But for some reason i cannot copy the first season. It always comes back with the error message. I checked my external hard drive, it has enough space, and is functioning properly. I haven't had problems with running iTunes on an external hard drive before. Any suggestions or solutions?