Before you connect your iPod to your Mac, you must prevent iTunes from syncing with your iPod. If it does, it might delete all the data on your iPod. At this point, your iTunes library on the Mac is missing some or all of the songs or other files that are on your iPod. If you sync your iPod with iTunes, you’ll end up with an iPod that’s missing the same files.
To prevent the iPod from syncing with iTunes:
Make sure your iPod isn't connected to your Mac and then launch iTunes.