Target Disk Mode
From Mac Guides
Target Disk Mode is available on most Macintosh models. It is a mode in which the computer boots into Open Firmware instead of the OS and essentially becomes an external hard drive. On all modern Macs, the connection is over FireWire. Target Disk Mode used SCSI on Macs in the mid- to late-1990s.
To activate Target Disk Mode, restart your Mac and hold down the T key during reboot. The screen will turn blue with a slowly blinking yellow FireWire icon. If your Mac is a portable, a crude battery monitor will also show up when the iBook or PowerBook is running from the battery. It is advisable to run portables off AC power when using Target Disk Mode for more than a few minutes.
Once in Target Disk Mode, this Mac can be connected to another Mac via FireWire just as you would an external FireWire hard drive — the internal drive of the Target Disk Mac will show up on the desktop/Finder sidebar of the Mac it's plugged into. However, only the first channel will allow for mounting of drives, and even then only the 'master' drive on that channel or, if SATA, then the only drive on that channel will be mounted.
Depending on the model, it is also possible to use the optical and other drives of the computer in Target Disk Mode. This can be used e.g. to circumvent the DVD region codes.
When finished with this disk, you must eject it like any other mountable volume. Once the Target Disk Mac is ejected, unplug the FireWire cable and hold down the power button to power it off — the next time it reboots, it will attempt to boot from the OS on the internal drive as usual, assuming there is a valid system installed there.
Using Target Disk Mode can be very helpful for transferring data quickly or for troubleshooting. It can also be potentially dangerous, however, as some of the sensitive Mac OS X files that are normally safeguarded by permissions and/or invisibility are much more vulnerable on a Target Disk Mac. Also, a Mac with FireWire running OS 9 will not be able to mount a Mac OS X volume unless that drive has OS 9 drivers installed. OS X can be installed onto a Macintosh started into FireWire target disk mode.