An iMovie library holds all the media and data used to make movies in iMovie. If your iMovie library is large and you want to free up storage space on your Mac, you can move your iMovie library to an external drive.
Before you move an iMovie library, it's a good idea to back up your Mac.
Moving your library to an external hard drive doesn’t affect any iMovie Theater content you have moved to iCloud.
You can store your library on an external storage device such as a USB Thunderbolt drive formatted as APFS or Mac OS Extended (Journaled). Apple doesn’t recommend storing iMovie libraries on external storage devices like SD cards and USB flash drives, or drives shared on a network.
You can’t store your library on an external drive or other disk used for Time Machine backups.
Include Movie Maker & Video Editor. Easy to make your own movie from images and video clips. Add animations and effects among images and videos. Custom and edit the music of your own video. Publish your video to Internet or DVD. Designed for Mac OS.
When your external drive is connected to your Mac, the library you moved to the external drive opens automatically in iMovie. If you disconnect the external drive containing the library, iMovie opens to a new, empty library created in the Movies folder on your Mac. iMovie automatically saves any projects you create and media you import in the new library.
If you want to open the library you moved to an external drive, you can open it after you reconnect the external drive. You can then move clips, events, and projects between libraries.