iPhotos Movies won't load after iPhoto Library moved to external harddrive

I recently moved my iPhoto library to an external harddrive (as it was getting too large). Since then, whenever I try select iPhoto Movies in iMovie my iMac freezes up. Not sure whether this is because:

- iMovie cannot find the moved library, or...
- iMovie is trying to reload hundreds of movie clips and freezing in the process

Is there a way to fix this, please?