Move to external drive

The external drive should be formatted as Mac OS Extended (Journaled).
FAT32 drives will not be visible to iMovie.

You will need to create a folder at the top level of the drive called iMovie Events. You can also create an iMovie Projects folder at the top level of the drive. If you make these subfolders of another folder, iMovie will not see them.

For best results, do all moves from within iMovie.
In iMovie, click View/Events By Volume.
Hold down the command key as you drag the Event icon in the Event Library to the icon for the external drive in the Event Library.

Also read about the File/Consolidate Media... Command in Help.

I should have said Events By Disk.
Events by Volume was used in an older version.

When moving your events, have this checked so you can see what you are doing.
When this is unchecked, you will see your entire Event Library organized by year (and month if you want) without concern about where it is physically located.

Thanks for this info, it's been useful!

One question - what is the purpose of holding down the command key while dragging? Or did you mean the option key?

By holding down the command key while dragging you get a MOVE. A simple drag will get a COPY.
If your objective is to free up space, use move. If you objective is to make a backup, or to share with a friend while keeling your own, or to use on two computers, use COPY.

Thanks. Bit of a finger fumble on my part I think, the first time I tried to use the command key it just selected multiple events. Then I tried again and it moved it (which is what I wanted). Thanks!