How do I export event video from iMovie 9.0.4 to my external hard drive?
Are you referring to an exported movie (i.e. all the bits stitched together) or just the events that you've collected that you would make up a movie project with?

If it is the former, then it is simply right clicking on the project and there should be an "Export movie..." option. I believe there is a menu bar item for this as well.

If you're referring to the latter, then I think it is as easy as just making a copy of the actual events from your hard drive to the external drive. You should be able to go to an individual event, right click and there should be a "Reveal in Finder" or some such option. You should then be able to just drag this event to the external hard drive (either as a move or a copy).

From memory, iMovie '09 doesn't care if you move the events or projects around on your hard drives. The only thing that might need attending to is that you'll have to re-launch iMovie for it to recognize that there is a movie or event on the other hard drive. You probably need to have the folders 'iMovie Events' or 'iMovie Projects' (can't remember how they're named right now, but you'll see them in your home directory) on those drives for iMovie to recognize it.

In short, you should be able to create a directory on your USB external drive like:

/Volumes/USB_External/iMovie Events


...and then copy whatever subdirectory from your /Users/myusername/iMovie Events/ folder to the external one.