sorry to post this as an answer but it's to long for a comment
These directions are taken from Apple.com.
Putting iPod into Disk Mode
"1. Before manually placing the iPod into Disk Mode you should verify
that it has a charge, if not you need to either charge iPod before
continuing or connect it to power. You can either connect it to a
high-powered USB port, or plug the iPod Power Adapter into an
electrical outlet and connect iPod to the power adapter.
2. Toggle the Hold switch on and off. (Set it to Hold, then turn it off again.)
3. Press and hold the Menu and Select buttons for at least 6 seconds
until the Apple logo appears.
4. When the Apple logo appears, release the Menu and Select buttons
and immediately press and hold the Select and the Play/Pause buttons
until the Disk Mode screen appears.
5. '''Disconnect iPod from the power adapter and connect it to your
computer; the screen on the iPod will change and say "Do not
If you are having difficulty putting an iPod model with a Click Wheel
into Disk Mode, set the iPod on a flat surface. Make sure that the
finger pressing the Select button is not touching the Click Wheel.
Also make sure that you are pressing the Play/Pause button toward the
outside of the Click Wheel and not near the center.
If you are still unable to put your iPod with a Click Wheel into Disk
Mode, use one finger from one hand to press the Select button, and one
finger from the other hand to press the Play/Pause button."
This of course can also be used in reverse to switch from Hard Drive
mode back to iPod mode.