By “Sandman68” at MacRumors
I know I am late responding and I am not sure if you have your answer yet, but here it goes.
iMovie stores data in two folders: iMovie Events and iMovie Projects. The iMovie events folder is larger of the two and contains the thumbnails and video. You can move the files to an external hard drive, but you can’t do it using Finder as you have realized. You need to do it from iMovie. 1. Open iMovie
2. Change the view in the Events window to View Events by Volume. You can do this by clicking the Volume icon to the right of Event Library or by right-clicking in the Event Library and selecting it.
3. Open the volume where your events are stored. Select it and drag it to the volume you want to store it in. This puts the event at the root of the volume in an iMovie Events folder. I have not tried to move it from the root. You also have to move each event one at a time instead of all on the same volume to the new volume. This would store your events to the external drive. The iMovie Projects folder which is relatively small would remain on your internal hard drive at ~/<home>/Movies/iMovie Projects