It sounds like you have your video files stored in the Photos application's library and when you imported them into FCP you chose the Leave in Place option tn the import dialog. If that isn't what you did please ignore the remainder of this reply.
If you haven't emptied the Trash, drag it (or use the Put Back command) to its original location. Open FCP and create a new library in the WD drive. Then se select the original library and drag the project to the new library. You will now have the project on two drives – both linked to your Photos library. With the new library selected, go to the Inspector and click the Consolidate button. This will bring copies of the actual media into the library.
Test the project in the new library to make sure it's working. If it is,open Photos and delete the movie. Then close Photos.
Re-opent the WD library in FCP and again test that the project is working.
Re-open Photos and from the File menu choose Show Recently Deleted. If you now delete the clips they will now be completely removed from your hard drive and additional storage space will be freed up..
It's actually preferable to avoid using Photos (or iPhoto) libraries for video.
Russ
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