Welcome to the community, Farahinzaimee!
I'd strongly recommend you to backup your movie storage from the external drive and copy it somewhere else before proceeding with the troubleshooting.
After you have backed all your data successfully, I'd suggest to run your HDD manufacturer's diagnostics tool and check up on the health and SMART status of the external drive. You should be able to find such a utility on their website, but in case you can't, here's a thread with some third-party suggestions:
http://www.tomshardware.co.uk/forum/282651-32-best-diagnostic-testing-utility
Another thing you should try is using a different USB cable and a different port on the computer. Moreover, I'd even test the external drive on a different computer, to see how it will perform there.
Keep me posted with the results. Hope this helps!
SuperSoph_WD