A few months ago, my Mac crashed and I had to restore the system from scratch. All the music on my computer was gone. But wait--all the music was synced up to my iPod, and with Firewire, I could have it back on my Mac in a flash. Well, not exactly. The folks at Apple worried that the iPod would be used to copy music illicitly between computers, so they disabled the iPod from being able to transfer music to a computer (even the same one from which the music came in the first instance!).
Finally, software programmers have come to the rescue.
Here's a wonderful article from Playlist magazine that tells you how to restore files to a computer's iTunes folder (either a Mac or a PC).
I used a program called Senuti. Here's how to copy files from your iPod to your computer with that program:
First, to restore the computer's music:
(1) Connect the iPod to your computer.
(2) Download and launch Senuti.
(3) Move the "library" icon into the "iTunes" icon in the left column of Senuti.
Second, to make that computer the home of the iPod, so that it automatically syncs up in the future:
(4) Open iTunes and confirm that its Library includes all the songs on the iPod.
(5) Click the iPod button on the bottom of the iTunes pane.
(6) Tell it to "Automatically update all songs and playlists"
(7) Confirm to the computer that you want to replace all iPod songs with those on the computer.
At least, this worked for me.