iMovie library taking up a lot of space (420+ GB)

There is probably no need to delete the Library.

As you edit, iMovie creates render files which are many times bigger than your normal camera clips.

You can delete the render files to free up lots of space. Should you need to revisit a project the render files will be automatically recreated.

To delete the render files select iMovie>Settings (or Preferences) and in the window that opens click on "Delete".

Another window will pop up asking whether you really want to delete them. Click on Delete again and the job is done.

Deleting the library would permanently delete all of the projects and the media contained in the library. You would not be able to access or re-edit any projects that you did. Further, you would permanently delete all of your source media unless you had it stored or backed up outside of iMovie.

If you do decide to delete your iMovie library, the next time that you open iMovie it will open in a new blank library that contains no projects or media. In other words, you start all over.

However, as suggested elsewhere in this thread the solution is to move your library to an external drive . The drive must be formatted Mac OS Extended (Journaled) or APFS. If you need to reformat, be aware that reformatting erases all data on the drive, so first back up any data that you want to keep.

-- Rich

You can copy that library to an external HD so you don't lose the data, then create a new library in Final Cut Pro and delete that old one by going to finder/ go at the top of the screen/ home/ find your library and delete it.

Once done be sure to empty your trash so it releases the space.

Keep in mind it may take a few for your storage to reflect it.