How to move movies already contained in library to external drive?
There's an easier solution to this than re-importing, and it works for both movies and songs (and probably everything else, but I haven't had reason to try it on any other media type).  

In iTunes, go to the File menu, then select Library, and then Organize Library.  On the resulting dialog, check "Consolidate files".  When you hit OK, any files that have somehow gotten stored in the wrong place will move back to where they belong on your external drive.  

After this has completed (it might take a couple of minutes, since I'm assuming that a movie file is big), you can test it out.  Command-click on your movie and select "get info", and then look at its file location on the General tab.  Its file location should show that it's on the external hard drive.  Then you can try to play it.  If it plays successfully, you can then delete it from your internal hard drive.