I keep all my music on my local drive, and all my movies on my 1TB external. Its easy to do, go to Preferences/Advanced, make sure your Music Folder library is set to where you want it to be. Then make sure that 'Keep iTunes Music folder organized' and 'Copy files to iTunes Music folder when adding to library' are both UNCHECKED.
Delete your movies from iTunes, saying NO when it asks if you want to delete the file itself. It will remove the reference of the file from iTunes but the files will remain on your drive.
Move the movies to your external drive, then drag and drop them back into iTunes. iTunes will now reference them in their new location (on your external drive), but they won't be copied into the iTunes Music Folder on your local drive.