i have all my songs for my ipod stored in my itunes folder on my laptop. the laptops hard drive is now nearly full though so i need to copy the itunes folder over to my external hard drive.
Just drag the 'iTunes music' folder to your external hard drive (copy it for now), using My Computer.
Then go into iTunes, remove all the songs, and then goto edit -> preferences, and in the advanced settings change the folder to be the one on your external drive. If you add all your songs again with file->add to library (choosing your folder on external drive), it should all be set.