In the course of creating a movie or trailer, you might use clips and photos outside of your library, especially if a storage device that contains media or iMovie libraries is attached to your Mac. If the media in a movie, a trailer, or an event is located outside your iMovie library, you can consolidate all the media in one location (either on your Mac or on a connected storage device) by copying the media to your iMovie library.
In the iMovie app 
on your Mac, do one of the following:
Consolidate source media for a library: Select a library in the Libraries list, and then choose File > Consolidate Library Media.
Consolidate source media for an event: Select one or more events in the Libraries list, and then choose File > Consolidate Event Media.
In the window that appears, click OK.
If a message appears stating that there is nothing to consolidate, all of your media files are already consolidated in one location.
In the iMovie app on your Mac, do one of the following:
If the project is not open: Click the Projects button in the toolbar.
If the project is open: Click the Projects back button in the upper-left of the toolbar.
Select the project you want to consolidate, then choose File > Consolidate Project Media.
In the window that appears, click OK.
If a message appears stating that there is nothing to consolidate, all of your media files are already consolidated in one location.
The project media is consolidated into the open library.