It is not a very well known fact, but all hard disks have a very strong hardware password capability build in. This password is usually stored both in a chip on the HD controller (the printed circuit board on the hard disk) and on the hard disk itself in a special hidden sector.
Setting this password will make the hard disk completely unusable to anyone that doesn’t know it. And not only on your computer, but on any computer.
A lot of newer laptops will set the HD password together with the BIOS password, completely locking all the hardware.