I'm no expert, but my understanding is that the difference is basically that bannock is closer to bread and hardtack more like a cracker, accounted for by the addition, or not, of baking soda. (Although, there are many bannock recipes without it.) Then again, someone else told me that it all comes down to whether you bake (hardtack) or fry (bannock.)

Salt is another key ingredient, it would seem. Bannock (often) has salt, which makes it more perishable. Hardtack doesn't, whioh lends itself to better storage and transit.

My Scotish ancestors my curse me for this but, in my experience, bannock, hardtack, fry bread, quick bread, ring bread, stick bread.... are all pretty similar, and their differences seem to come down to the recipe and cooking methods used.