This is a silly work aroun, but if you use "Symbolic Linker" you can make a link from the external hard drive to your movies folder and iMovie will store and pull from the external. For example, I copied my Movies folder to my external HDD, then used the "Symbolic Linker" program to make a new link of the movies folder on the external drive. Then I copied the link into my home folder on the internal drive and it worked! All my data is now stored on the external drive for everything in my Movies folder. You could go one step deeper and just choose the iMovie projects folder. Good Luck!!