With these steps complete, your iTunes library files are all in one place and are ready to be backed up to your external hard drive.
Begin by quitting iTunes.
Next, locate your external hard drive. You should be able to find it on your desktop or my navigating through My Computer on Windows.
Then open a desktop window and navigate through it to your iTunes library.
On Windows, it will be in your My Music folder, which you can reach through the My Documents folder or, on Windows Vista/7, by double clicking on your hard drive.
On a Mac, it will be in your Music folder, accessible through the sidebar of the Finder window or by clicking on your hard drive, selecting Users, and clicking on your username.
When you've found your iTunes library folder, drag that folder to your external hard drive. This will begin the iTunes backup. The amount of time the backup will take depends on the size of your library. When the transfer is done, your backup is complete and your external hard drive can be disconnected.
Making periodic new backups can be a good idea if you regularly add content to your iTunes library.
Got a tech support question? Ask it in the discussion forums.
Want tips like this delivered to your inbox every week? Subscribe to the free weekly iPhone/iPod email newsletter.