Next on the agenda will be "primary" and "secondary" since the connotation is that one is superior, in some manner, to the other. We certainly can't have that thought running around in the common folk's head.
Legal Disclaimer: The necessity that one hard drive must, in some manner, be subordinate to another hard drive is not intended to address the circumstances of, or offend, any particular individual or entity and does not necessarily reflect the social mores of the manufacturer, its employees, heirs, and assigns. The manufacturer shall not be responsible for any angst, offense, or loss of self esteem due to the necessity of enabling multiple hard drive relationships. No one should enable a dependent feature without appropriate advice from an emotional care professional.