just save a copy of the project file onto the external drive (file–>save project as). Take the drive to the post house and you’ll be good to go. But, make absolutely sure that you have transferred ALL the media to the external drive….this includes still images, background/graphics from a third party, etc. Too often do I use still images that are saved to a folder on my desktop and forget to grab them when moving my drive to a different machine.
It should be noted, if they are using a newer version of FCP, you won’t be able to open the project on your older system….this isn’t microsoft word.