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 uploaded to iCloud.
Prepare your drive
You can store your library on an external storage device such as a USB drive, FireWire drive, or Thunderbolt drive formatted as MacOS 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.
Move a library to an external storage device
- Quit iMovie
- Select your external drive in the Finder, then choose File > Get Info.
- If the information in Sharing & Permissions isn’t visible, click
, then make sure the “Ignore ownership on this volume” checkbox is selected. If it's not selected, click 
to unlock it, enter an administrator name and password, and then select the checkbox. - Click on your desktop to make sure that you’re in Finder, then choose Go > Home from the menu bar at the top of your screen.
- Open the Movies folder, then drag the file named iMovie Library to your external hard drive.
- When the library file is finished copying to your external drive, double-click the library file to open it in iMovie.
- Make sure the library you moved contains all your media, then drag the original file in your Movies folder on your Mac to the Trash.
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.