To use FireWire target disk mode
1. Make sure that the target computer is turned off.
2. If you are using an Apple portable computer such as a PowerBook or MacBook as the target computer, plug in its AC power adapter.
3. Use a FireWire cable to connect the target computer to a host computer. The host computer can be powered on.
4. Start up the target computer and immediately press and hold down the T key until the FireWire icon appears. The hard disk of the target computer should become available to the host computer and will likely appear on desktop. (If the target computer is running Mac OS X v10.4 Tiger, you can also open System Preferences, choose Startup Disk, and click Target Disk Mode. Restart the computer and it will start up in Target Disk Mode.)
5. When you are finished copying files, locate the target computer's hard disk icon on the desktop of the host computer and drag it to the Trash or choose Eject (or Put Away) from the File menu.
6. Press the target computer's power button to turn it off.
7. Unplug the FireWire cable.
See Apple Tech Notes: http://support.apple.com/kb/HT1661