If you signed in to iCloud, then video clips, trailers and movies shared to iMovie are automatically uploaded to iCloud. You need to check mark "Automatically upload content to iCloud" option located in iMovie. If you delete a video clip from iCloud, it will remain in the Theater. If you sign out of your iCloud account on your Mac, then all of your iCloud video clips disappear from the Theater. Whenever you signed in, the video clips reappear again. But they are not deleted. To delete a video clip from iCloud, open "iMovie" on your Mac and click "Theater" button located in the toolbar. Select the video clip which you want to delete and click "Delete" button. A dialog box appears, click "Delete from iCloud" button. This is how you can delete iMovie Theater video clip from iCloud.
Quick Tip to ensure your videos never go missing
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