To share music back and forth between your iPod and a computer (Mac or PC) you need to set your iPod to update manually rather than to sync automatically with iTunes. Manual updating allows you to delete songs in iTunes while keeping them on your iPod, or to keep songs in iTunes and remove them from your iPod.

To set up your iPod so that you can manually add and delete songs:
  1. Connect your iPod to your computer and open iTunes
  2. Select your iPod in the iTunes Source list
  3. Click the iPod Options button in the lower-right corner of the iTunes window
  4. In the iPod preferences window, select the Manually manage songs and playlists option and then click OK
To manually add songs to your iPod:
  1. Drag the individual songs or playlists from the iTunes window to the iPod icon in the Source list
  2. The iPod icon will flash red and the iTunes window will show Updating iPod while it is updating
  3. When you are finished, click the Eject button next to the iPod in the iTunes source list
To manually remove songs from your iPod:
  1. Select the iPod in the iTunes Source list
  2. Open the iPod's contents in iTunes
  3. Within iTunes, select the iPod’s song or album you wish to remove and then press the Delete key
  4. iTunes will display a warning to make sure you want to do this - click on OK

If at some time you wish to restore songs from your iPod back on to your computer, it is important to note that the ability to copy songs from your iPod back on to your Mac or PC is not supported in iTunes.

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