Hard Drive Passwords
Some hard drives support passwords. These passwords can be implemented in computer's operating system, its BIOS, or even in the hard drive's firmware. Passwords implemented in the OS are the easiest to remove, those in the firmware are the hardest.
Sometimes people use the term "password" but the hard drive is really encrypted, and the password is used to unlock a decryption key. These passwords cannot be removed---the encryption key must be cracked or discovered through another means.
Disklabs (www.disklabs.com) is able to remove some forms of hard drive passwords.