SOLVED Film won't drag and drop to external hard drive

[Laura Macdonald]

I have a mid 2014 MacBook Pro and have downloaded a film of about 7.5GB. That's using up quite a lot of storage so I tried to drag and drop it in to an external hard drive that I have with lots of storage (about 130GB available) but it won't "stick", it just slides back to it's position on the desktop. Why? Thanks.