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.
The hard disk manufacturers are unable to unlock a password protected hard disk, as there aren’t any “secret” master passwords build into the firmware. Even swapping the controller of the password-protected hard disk with exactly the same controller from an unprotected HD will not remove the protection on most disks, as the password (together with most of the firmware) is also stored on the hard disk itself.
The only way of retrieving any files from a password protected hard disk without knowing the password is to send it to a data recovery company for unlocking, but not all data recovery companies could or would unlock a password protected HD.
In this time of rising identity theft, protecting your personal data by locking your hard disk with a password is indeed a good idea. The downside is that when the HD eventually malfunctions it will be harder or even impossible to retrieve any files from it. So, the first rule of using a computer applies here in full strength – “Your data is only as good as your latest backup”!