By default, your iPod is set to “Automatically sync songs to my iPod” also called “Automatic mode”. In order to transfer music from multiple computers to your iPod, the device must be set to “Manually manage music” also referred to as “Manual mode”.
When your iPod is set to “Automatic Mode”, iTunes automatically updates your iPod’s music library whenever you connect your iPod to your computer. iTunes transfers new songs you’ve added and erases songs you’ve removed. The first computer you connect your iPod to is its “home” computer. The music library from that computer is copied to your iPod.
When you connect your iPod to a different computer, an alert box appears with this message:
If you want to delete the music library on your iPod and make the new computer its “home” computer, choose Erase and Sync. iTunes will delete all songs and playlists on your iPod, and then will copy the music library and playlists from the new home computer to your iPod. If you have used CopyTrans to transfer all your music to iTunes and are seeing this message, please make sure your music is playable in iTunes first, then accept the message. This is the normal procedure and will only need to be done once.
You change the iPod synchronization mode to manual mode in iTunes:
Open iTunes on the iPod’s home computer.
Select iPod under Devices in the menu on the left-hand side of iTunes.
Click the Summary tab.
Click “Manually manage music and videos” to enable that option.
Click OK in the resulting dialog box.
It is normal for the iPod to take a few seconds to change from automatic mode to manual mode.
When in manual mode, to add songs or playlists, drag them from iTunes to the iPod icon in the sidebar. To remove songs or playlists, select them on the iPod in iTunes and hit the delete key. You can also create playlists directly on the iPod.
Note: the iPod shuffle is intended for use with a single computer. You cannot load music from multiple computers or iTunes libraries onto iPod shuffle like you can with other iPods.