Refer to the attached picture for the following...
If you see at lease one white square (upper right, four squares), those are items you have used the Share menu on that get written to the project folder, so you can actually play them. It seems items shared to iTunes are placed in a sub-folder of a folder named 'iMovie Sharing'. I haven't clearly figured out the sharing fully.
If you see a yellow marker (lower left), that means you do not have access to the assets. You'll need to copy the appropriate files from the 'iMovie Events' folder. If you haven't shared the video, the preview window will be black with text near the center that says 'Source clip is missing'. Also, you will not be able to find it in the 'Event Library' pane.
If the assets of the project are not in the events folder, like from iPhoto or else where, I think you'll want them always on your USB drive. I haven't found a way for iMovie to re-aquire some assets such as items dragged in to it.
Check out
this response regarding the layout and my thoughts at the time.
P.S. When I tried a USB stick, iMovie didn't want to see it. I'm not sure why. Perhaps a speed issue?