Depending on which version of iMovie you have it could be very easy, when you attach the external hdd to the mac it will appear in iMovie as one of the 'locations' where projects/events can reside. And you would move them ONLY from within iMovie, not outside of it. This will help keep any current projects and the events associated with them from breaking.
This is how it would work, attach the external hdd. Open iMovie go to the Projects Browser. Make sure you can see the external hdd in the list of devices. You then can drag the project directly over the icon of the external hdd. iMovie will ask you, do you want to move Events associated with this project? Go ahead and say yes and it will move the Events over too. So you might ask what about the events not already in a project? Well you can do the same thing down in the Event Browser too. Only you want to make sure iMovie knows you want to see all the hard drives attached to your computer not just the internal one. So there's an icon in the top right corner of the Event browser called Group Events by Disk. It has a cartoon icon of a hard drive on it. Just click it once to turn this on. You should now see the external hdd in the list. Drag all your Events onto that disk. So now for the moment of truth,. . .
Go up to your project browser and open up one of the projects FROM the external hdd and skim it, edit, add do something with it. Switch to a different project on the external try the same thing. Go down to the Event browser and check out some events on the External HDD as well. It should all be the same as it was on the Internal HDD. Last Step: Delete the now duplicate/original Events/Projects on the Internal HDD. You have perfect working copies on the external and do not need to keep the originals on the Internal HDD. Any time from this point forward you will only open Events/Projects from the external hdd and save back to that location. And iMovie will take care of the rest of it for you.
