I learned the hard way that disconnecting my iPod Nano while it was still connect to Mac would freeze it. If you ever face a frozen iPod Nano, you need to toggle the ‘hold’ switch on and off to ‘lock’ then ‘unlock’ the iPod controls, then press the ‘Menu’ and ‘Select’ (centre) buttons for about 10 seconds until the Apple logo appears and your iPod Nano restarts.