The important thing is to do everything from within iMovie and not with the Finder.
- Create a new library on your external hard disk (which must be Mac-formatted - HFS+ journalled).
- Copy (not Move) the events from the old to the new library (so that the library on your internal drive is preserved until you have made sure that everything is fine with your new library on the external drive).
For full details see: http://help.apple.com/imovie/mac/10.0/#mov3fa25bae7
Its possible that the new new library still references files on your internal disk. To make sure it contains actual copies of all files that are required it is advisable to select your new library and Consolidate Media. See:
- If everything is working fine in the new library you can delete the old one.
I've finally got my iMovie Library on to and external drive. I did it myself but was too scared to delete the one on the HD without someone to check everything was OK (comes with being an old granny). The Genius Bar did this, and showed me how to check each item in parallel.
I only lost the project - the one I was working on. It somehow disappeared in the ether. Everything is working well.
Fortunately, there doesn't seem to be the blacking out of clips, either. Freeing up some space on the HD must have helped.
Thank you again.