As long as you are a subscriber different subscribed 722s can play the content. There is a whole house key that is unique to your account that is given to your subscribed 722s/622s/922s that allows them to share the same encrypted drives. If you take your drive over to a friends house or buy another receiver and do not subscribe the new receiver it will not be able to decrypt the external drive.
So, if your 722 breaks you can exchange it with Dish and your external hard drives will work with the replacement receiver, once the receiver is activated on your account it will be given the decryption key.