Saving an Imovie project to an external Drive

Yes. Go to your movies folder and click on the iMovie library icon to select it. Do an Edit/Copy and then open your external drive and Edit/Paste the library into your external drive. iMovie will refer to the library on the external drive where you can work on and store your projects there. You should leave the iMovie app itself on your internal drive. If all is functioning well, you can then delete the library that is on your internal drive.

Before you move your library to the external drive the external drive must first be formatted Mac OS Extended (Journaled) or APFS. If you need to reformat, be aware that reformatting will erase all data that is on the drive. So back up elsewhere any data that you want to keep.

Here is a link to an online help menu that has a good search feature. It will help you get familiar with the features of iMovie 10.

https://help.apple.com/imovie/mac/10.1/

— Rich

Would you like to create future libraries on an external drive or somewhere other than your Movies folder?

If so, follow these simple instructions before you start a new project.

https://www.picbackman.com/tips-tricks/how-to-create-new-imovie-library-on-external-hard-drive/