iMovie missing remote media again!

I have produced at least 30 videos on iMovie with good success. However this video I have been working on for two months keeps giving me trouble! First, half of the photos that were downloaded to the timeline disappeared and the "missing remote media with ?" appeared, so I dragged the missing 80 photos from Photos to the timeline. I returned to the project a few days later and again the same 80 photos were missing! I copied the photos again to the timeline and this time saved the project on an external drive as a file. When I opened the project again 80 or more photos were missing with the same "missing remote media?".

I don't want to have to copie the missing photos again and have the same thing happen again. When I try to drag the saved video from my external drive to my hard drive it won't allow that. Should I drag it to the desktop and then to iMovie? I don't know what to do next!

What is going on? Is there a software issue? I am using an iMac 27 inch, macOS Ventura 13.7.7.

Please help.