iMovie Library location is by default located in your Movies folder.
To move it an external hard driver or a server, open your finder window and drag the iMovie library file to another location. Make sure your iMovie is not running before attempting to move the iMovie Library file. Unexpected things my happen.
After the iMovie library file is moved to another location, just double click the file to open iMovie and it will automatically update with the new location for the library file.
Thats everything you need to do to move the library file of iMovie.
To read more about iMovie visit Apples official iMovie support pages.
Happy filming!
About Author
