Issue with downloading Movies in iTunes on Mac

Hi all,
I've recently been experiencing a problem in iTunes on my mac.
I'm running the latest version of iTunes, with the latest version of Mac OS. I've purchased a movie, which I believe is over 4GB in size, however, whenever I try to download it, as soon as I press download, I get an error 1309.
I've researched it, and on forums I saw e possible solution, which was, if you have a iTunes media folder on an external drive, which I do, change the folder to the mac media folder, then download the movie, and change the folder back to the media folder on the external drive. I've tried that solution, but it didn't work, I now get an error 50.
Could anyone help with this?
Cheers,
Hubert