Target disk mode is a handy way to quickly transfer big files between a laptop and desktop Mac. You simply connect the 2 firewire ports, and restart the laptop while holding down the "T" key.

If the laptop disk does not appear on the desktop of the host Mac, there are a couple of things to check.

Apple recommends not having any other firewire devices connected to the host at the same time, but I've found that even if you forget, it still works.

However, the chief reason this has not worked for me is ill-fitting FireWire plugs that aren't plugged all the way in on the laptop. Another thing to try is unplugging and replugging the firewire cable: Mac OS X 10.2: FireWire Target Disk Mode Requires Hot Plugging for Some Computers