Moving entire Movies Folder (including Events, Library, Projects, etc.)

Have you also checked that you do not have a lot of duplication?

If you have placed clips in the Movie folder and imported them

into iMovie, iMovie will copy those clips into the iMovie Library.

To verify, right click on the iMovie Library and select show package

contents. Each project should have its own folder. In those, open up the

"Original Media" folder and see if any files are duplicated elsewhere.

One thing some people don't realize is that iMovie projects create their own

"little world" with copies of any media that is else where. The "Original Media"

will be unaltered copies of content that may be stored else where.

It is best to leave "working" files on the computer and just move the "referenced" (library) to an external.

However, if you are done with the project/event, then I'd archive it and move it permanently and separately to an external for possible future use. I burn a Video_TS folder of my projects and then move that to external storage; after that, I delete any files no longer needed. Videos take up an inordinate amount of space and need to be managed continuously. Please note that while editing and rendering video, you should have at least the same amount of empty hard drive space as your project/event files for the processor to work efficiently - I can easily amass 100+ GB of temp files for a one hour HD project, so I keep a lot of empty hard drive space available (far more than the minimum 10 - 15 GB generally recommended).

The iMovie Library should be the bulk part of your iMovie Folder. If you have other files outside of the iMovie Library, you can simply drag n drop them to a folder on the external drive and then remove them from your internal drive

check out > Move your iMovie for Mac (2014) library - Apple Support

Follow the directions and you should be just fine.