How much disk space does the backup facility use?
The backup facility makes a copy of your iPod database, not the songs themselves. If your iPod was a book, think of the backup as taking a copy of the book’s index, not the actual content. This being the case, the backup takes up very little space, but will depend upon the number of songs you have stored on your iPod. The backup is stored on the iPod, not on your computer.
