The enclosure holds the little piece of computer code that runs the mirroring and small amount of software that you can access on the drive. (e.g. the web gui).
The linked piece tells you that the enclosure holds a master key which has encrypted the drives. The drives are permanently encrypted regardless of whether you have told the drives to be or not. All you are doing when you set a master password and “tell” the drives to be encrypted is you are encrypting the master key - thereby ensuring that no one can hack the drive enclosure and retrieve the key and then decrypt the drives. Therefore, the drives are always encrypted and if there is a failure with the enclosure then you need to find out a way to get that key and somehow use that master key to decrypt the drives.
I am so happy that I stumbled across this article while looking for something entirely different ! I shall be dumping my HD book duo immediately (after having carefully taken everything off and put it somewhere that I know I can get into regardless of enclosure hardware failure !!). Well done @jtech !
