There are a couple of ways to do this.
1) You could move it in the Finder, but ONLY IF YOU HAVE CREATED NO PROJECTS WITH THESE EVENTS. If these Events are referenced in any projects, you need to do the move in Imovie, not the Finder.
2) One strategy would be to move the Events only to a single drive. To do this, in iMovie,
click VIEW/EVENTS BY DISK. You should see all your properly formatted disks in the Event Library. Hold down the Command Key as you drag the small icon for the Event to the small icon for the external drive in the Event Library. (Note: If you do not hold down the command key, eht Event will copy, not move.
3) Another strategy would be to keep projects and events together. This way, if you ever needed to edit the porject again, all assets would be in one place. To do this, click VIEW EVENTS BY VOLUME.
Move the Project to the drive where you want it to be by command dragging the icon in the Project Library to the icon for the external drive in the project library. Once the Project is in its final resting place, click FILE/CONSOLIDATE MEDIA. This will let you Move all Events that are reference in the Project. It will also let you copy in music, photos, etc. that are used in the project so that everything is in one place.