Not sure specifically where, but right around there is where firewire target disk mode was added to the firmware. It may only support the master boot drive, not slave drives or the optical drive.
If it's not available for your firmware, you'll need to remove the hdd and do a "from another volume on this computer" and plug it into the usb/firewire. I keep one of those generic ide/sata to USB adapters in my laptop bag for just such an occasion.
