I've run into this more than a few times, so I thought I would post a work around (aka hack) for setting the iTunes sync mode to manual. Most people may never need this, but it is really helpful is some/all of your music/videos are hosted on an external or network drive.
To override your iTunes and iPod sync settings and force iTunes to start in manual sync mode, hold down Command and Option (Mac OS X) when you connect your iPod and keep holding it until iTunes starts.
(Windows PC users – hold down Control and Shift and keep holding it until iTunes starts)
This will force iTunes to initiate the iPod connection wizard. You will be prompted to confirm the name of your iPod. Simply deselect (un-check) the option for "Automatically update" your iPod.
By over-riding the iTunes automatic sync option, you can keep everything that is already on your iPod and easily add songs, movies, etc that you have picked up while your away from your external hard drive or network music storage.
This is also pretty handy if you just want to charge your iPod using the USB or Firewire cables, but don't want to sync any music/movies/etc.
To return to automatic sync, simply select your iPod in the iTunes source pane, select the Music tab, and check to automatically sync your songs.