Question: when I import videos from my video camera, the videos appear to get imported, and I can put them in my timeline. When I re-open iMovie at a later date, the videos are "missing". Repeating the process gives the same result.
It appears as if the import is only saving placeholders and the video camera has to be connected all the time for the import to work. I am using iMovie 10.1.8 and MacOs 10.13.1. I didn't have the problem importing still photos from my video camcorder.