I’ve been trying to get a movie out for days now with iMovie. There is a strange interaction with Time Capsule backups where with something as long running as an iMovie creation (in my case 7 hours to make an hour plus HD movie), so that Time Capsule tries to backup the file and thus you end up with a conflict. The fix is to either turn Time Capsule off with iMovie is creating or run

This requires using the Terminal app, in your Applications/Utilities folder

 

To turn off Time Machine snapshots type in the following command:

 

sudo tmutil disablelocal

 

The Mac will likely ask for your password, so type it in and hit the return key. The type the word, exit and hit return. Quit Terminal.