Media libraries grow large and hard drives can seem to shrink as thousands of songs and hundreds of videos begin to fill up the space. You may, in fact, be thinking of getting a big external hard drive to use for iTunes storage. That's just dandy, but you need to make sure iTunes knows what you intend to do.
If you rudely drag the iTunes Music folder to a different place without telling iTunes, it thinks the songs and videos in your collection are gone. The next time you start the program, you'll find it empty. (While iTunes remains empty but calm, you may have heart palpitations as you picture your media collection vanishing in a puff of bytes.)
To move the iTunes Music folder to a new drive, just let the program know where you're putting it. Move the folder to the desired location, then, in the Preferences box (Ctrl+comma/⌘-comma), click the Advanced icon or tab. In the area labeled "iTunes Music folder location", click the Change button, and navigate to the place where you moved the folder. Finally, click OK.