Eu não sei como explicar melhor do que esta parte de um artigo que eu vi ...
An SED is a self-encrypting hard drive with a circuit built into the disk drive controller chip that encrypts all data to the magnetic media and decrypts all the data from the media automatically. All SEDs encrypt all the time from the factory onwards, performing like any other hard drive, with the encryption being completely transparent or invisible to the user.
To protect the data from theft, the user provides a password. This password is used by the drive to encrypt or decrypt the media encryption key. In this way even the media encryption key cannot be known without knowing the password.
Extrato retirado de link às 2:35 am 10/01/2015 GMT + 11: 00.