Rich, I've been successfully able to import, edit (nothing fancy; pare video down by about 70-75%, put a cover title and some simple transitions) and export (to mp4 on a backup hard drive) three VHS tapes of 1980's movies, each about 2 hours long and about 30GB in size, albeit with much googling and finding help with your answers to others' queries (such as the rendering 10008 issue) and I have about 30 more hours to go till I'm done, which I was hoping would be within the next two weeks.
They are all in .avi format. Today, iMovie and I *really* didn't get along. Each movie (large ones like above, but also some only 900MB) that I imported, only imported with a blue screen (no video) but did have audio. But each and every time I tried to do something in iMovie after importing such movie, it would totally freeze. I restarted iMovie countless times. I tried hours of things that your posts, other posts and Apple suggested (restart, reset NVRAM, deleted and reinstalled iMovie, tried a guest user, tried a new user, tried a new library, tried copying the video file to desktop and trying to import from there instead of the backup hard drive all the old videos are on, etc. Nothing worked. As a test, I tried importing all three of the movies I have successfully imported, with one working (importing with both audio and video) and two doing the blue screen thing (no video but had audio), so that didn't help me troubleshoot.
Any help you are able to give me would be appreciated very much. I just realized that perhaps I should have started a new thread for this, so I hope this is okay.