So what's the status now? Are these movies on the WD and have they been added to your library or do you just have a folder with movies on the WD?
You can add items stored on a different drive to iTunes and not have the files relocated by holding down the option key while dragging the items from the drive to iTunes. While this works it can have disadvantages, particularly when it comes to moving your files to another drive at some stage. If your external drive is not connected and you want to use iTunes to only play music you will still see a bunch of broken link symbols ! until you restart iTunes with the drive connected.
An alternative is to create a library just for movies. While some don't like this I know I only watch a particular movie only once every 5 years or so. I don't need to have it in front of me in my library all the time where it takes up a small but definite amount of resources to manage as part of my general music library. If I did feel in a movie mood I could restart iTunes with the movie library and browse that. The external drive only has to be attached when I want to use that library (I actually have 4 libraries on my computer for different purposes since some of them I don't need to see the content unless my mother in law wipes her mp3 player accidentally 🙂 ).
To create a new library start iTunes while holding down the option key and tell it to create the new library on the external drive. Add media to it by dragging media to the 'automatically Add To iTunes" folder you will find in the new iTunes folder on the external drive.
All the above said, I have memory of some people complaining about certain external drive models and AppleTV. I don't have an Apple TV so I didn't bother reading or remembering them but you might want to do a web search and see if there are any issues with a WD MyCloud.
