Thanks! Great info. I used iMovie to move all of the projects and iMovie events to the external drive and then plugged the external drive into my new mac and opened an iMovie project. Everything looks fine. I'm not sure if I am fine or if I need to do it the way described in your last section at the bottom:
If you want to move Events, and copy photos, copy music, etc for editing on another Mac, you should use the FILE/CONSOLIDATE MEDIA feature after moving the Project as above.
Also, now that I have my iMovie projects and events on the external, I am going to need to backup that drive to another external drive. So can I just copy one to another? And if the primary drive fails, will the backup drive work fine even though I didn't use iMovie?
The File/Consolidate Media command is useful for putting copies of any photos or music on the hard disk without moving your whole iPhoto library or your whole iTunes library to the other Mac.
If you have moved these anyway and are satisfies that your projects are working correctly, there is no need to Consolidate Media.
I haven't tested your second question so I can't be sure. I think it will work if the projects and events are on the same drive. Just in case, I would jot down the name of your Volume that holds the Events and Projects. If you backup volume fails to work, I would try renaming the Volume to the one iMovie is expecting, but I doubt this will be the case.
Another way to be sure would be to do the backup from within iMovie.
If you are using SuperDuper! or Carbon COpy Cloner for backup, I suggest your test your backup in iMovie so you can see that everything is well.
Personally, I back up all projects. I backup original events of all my DV. For AVCHD, I back up the Archive copies, but not the Events. I can always re-create the Event from the Archive.
Always test your backup plans.