Is there a way to delete unused media in an iMovie library?

Clips that are used in a project have an orange line across some or all of the bottom of the clip thumbnail. That's the only way I know of to tell whether or not a clip is in use. You can select multiple clips that are not in use and delete them as a group but AFAIK there is no way to create a list of all unused clips.

You might consider previewing & culling your shots before importing them into iMovie. Many photo organizer apps can also handle video files ... Photo Mechanic, for example, handles AVI, MOV, Mpeg 2 and MP4 video files. So do Adobe Bridge, LIghtroom and other apps. Don't be put off by their emphasis on photography, they can work with video also.

I did some poking around and found this video that demonstrates a way to delete all unused clips.

https://www.youtube.com/watch?v=Jrt_7YM1nHQ

What it shows is that instead of finding all Unused clips, you select ALL clips and delete them but when the delete happens clips that are still used in any Projects in the LIbrary are not deleted. So you are left with only clips that are in use.

It's a backward approach but seems to work. I'd try it with a throwaway iMovie Library to verify that it actually does what is shown. If it works, it's the answer to your question and you don't have to do any searching for unused clips.

l008com wrote:

The problem is, if i use 3 seconds out of a 3 minute clip, that orange line will be essentially invisible.

The other problem is, there are 354 clips in the media for this video, and I'm still filming, so there will probably be close to 400 by the time i'm done.

IMHO that is way beyond anything iMovie was designed to do. I would expect Final Cut, Davinci Resolve, Avid Media Composer or Adobe Premiere to be more suitable for that level of editing.

If you want to delete unused clips from your iMovie Library, see https://discussions.apple.com/thread/255486092?sortBy=oldest_first

Just make sure they are truly unused clips. Once you put a clip in a project timeline, your project is dependent on the entire underlying imported clip no matter what you do to it in the timeline including removing sections of the clip in the timeline.