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.