iMovie hangs on new top-of-line Mac

Correct. Deleting preferences in iMovie is perfectly harmless. That's not what is causing the problem.

If your iMovie library is on an external drive formatted for Windows, i.e, FAT 32, you need first to back up the drive's data elsewhere and reformat the drive Mac OS Extended (journaled). Formatting a drive erases everything on it so that's why you need to remove and back up any data you want to keep.

I am not sure what you mean when you say "when I try to import iMovie Library". There should be no importing involved because you only have to click on it to open the library. Are you referring to updating projects and events from a prior version of iMovie, iMovie 9? Those projects would not appear in current version iMovie 10.1.8 unless you do a File/Update Projects and Events from within iMovie 10.1.8.

In any event, try recovering your projects from the iMovie backups folder. To get to the backups folder, follow this file path: Go/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovie Backups. When you get there you should see a list of past library backups. Click on one that predates your issue and iMovie will open in the backup library. Hopefully you will find your older projects there.

-- Rich

Sometimes preferences get corrupted that cause iMovie to function improperly. Try opening iMovie while holding down the Option and Command keys, and selecting to delete preferences in the box that appears. See if that solves it.

-- Rich

O.K. Skip Home and try this, and follow the path from there:

You are welcome. Sometimes hands-on help works best.

-- Rich