Pick up your dad's powerbook, easily access entire HD as target. No muss, no fuss, no permissions required.
Anyone who picks up any laptop can do that. All that's required is to boot from a CD that gives you admin permissions. Or take the HD out and put it in another system.
Even easier, connect a USB drive, boot from it, and the whole machine is yours. You don't even need another system involved like with target disk mode.
Whenever you lose control of your laptop your data is compromised. The best protection, as mentioned previously, is to encrypt any critical data.