This is usually caused by some other program accessing the iPod drive. You can try to remedy it by shutting down any programs that could be accessing the iPod drive such as anti-virus programs. But sometimes even that won’t do it.
However, there is still hope. When presented with the ‘Can’t Unmount iPod’ error, it is OK to disconnect the iPod from the computer. The iPod will then flash an icon indicating that you should connect the iPod to a FireWire plug.
Make sure that when you plug it back in, the connection you connect it to supplies power to the iPod as it needs external power to finish the Restore/Update process. A progress bar will appear on the iPod and when done the Updater program will indicate that the Restore has been successful.