I know this might seem ridiculously obvious to some but if you need to copy a bunch of movie files from an external drive to your iPad and you don't have space on your laptop, this approach will work.
I use a laptop as my primary iTunes syncing device and even though it has a 300 gig drive, movie files eat that up fast so I use an external Drobo with 6TB of space to hold large media.
If you want to copy your videos to iTunes but don't want to copy the movie file to the local iTunes collection, go into iTunes, Preferences and under the Library tab *uncheck* "Copy media when adding to iTunes library". Then drag your movies directly from the external drive to your iTunes library.
Click on the iPad when connected in iTunes and choose the Movies tab. Check the movies you want on the iPad and click apply. This will copy the movies to the iPad.
Remember to re-check "Copy media" option above if you want this to be the default when adding music (I do this because it's convenient).
If you disconnect the drive and re-sync it throws an error but the movies stay on the iPad and you don't lose all your laptop hard-drive space.