During a disk tidy I moved a whole load of iMovie Events from one disk to another (from within iMovie). This worked fine.
However - I had some external media (mp3 file, couple of stills) which I'd just dragged in from the Downloads folder - and having moved these I found that iMovie no longer could find them (yellow warning triangle on the audio media and black video where the stills were used).
To reattach these takes a little fiddling.
The information is in the imovie project file in the file called Project. This is a binary plist file.
Make sure that iMovie isn't running and you'll probably want to backup the Project file.
So at the command line I did:
cd Movies/iMovie Projects.localized/<ProjectName>.rcproject plutil -convert xml1 Project
Project is now an xml plist and you can edit it. I opened it in textmate and then searched for the old path (in my case Downloads) and then replaced with the new one (it didn't like spaces in the path - but - instead of trying different escape syntaxes etc I just used a path with no spaces). I expect you can edit it with the standard plist editor too - but I didn't try.
Then once complete:
cd Movies/iMovie Projects.localized/<ProjectName>.rcproject
plutil -convert binary1 Project
This should now allow iMovie when next opened to find the missing resources. At least it did for me.