If you go this route, MAKE SURE you eject the laptop from the desktop and power down before you unplug the FW cable between machines.
A disk with a system on it can get scrambled pretty bad if it is disconnected while still mounted. Powering down is just my own added super-cautiousness.
I had my laptop in target mode attached to my desktop and the cable was accidentally pulled by a curious kitten. My laptop refused to boot after that. I’m not sure what my technogeek did to get it back, but it was a miserable couple of hours waiting while he sprinkled magic geek dust on it.
Now I only use target mode under extreme circumstances.
debe