Answer from iMovie help menu:
If you want to save space on your computer's hard disk, you can move your stored video in your Event Library to a compatible external hard disk connected to your computer with a FireWire or USB 2.0 cable.
You can also copy Events to another hard disk. You might do this, for example, if you want to create a duplicate copy of your video library or an Event on a friend's hard disk.
NOTE:When using an external hard disk with iMovie, make sure it has been formatted as Mac OS Extended (Journaled).
To copy or move one or more Events to another hard disk:
Connect a compatible external hard disk drive to your computer using a FireWire or USB 2.0 cable.
In iMovie, click the Hard Disk button to see all available hard disks listed in your Event Library. The button is located in the upper-right corner of the Event Library.
Select the Events that you want to move or copy, and then do one of the following:
To copy the Events, drag them to the icon of the hard disk where you want to copy them.
A copy of the Events remains in the original location, and any projects using clips from the Events still refer to them at their original location.
To move the Events, hold down the Command key as you drag the Events to the icon of the hard disk where you want to move them.
The Events are moved and deleted from the original location, and any projects using clips from the Events refer to them at their new location.
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