I guess I read too much too. I have heard of it, never tried any. I will need to give some a try.
Of interest, if you look on the back of Hawaiian sweet rolls, it says that it's history is as hard tack. Basically, it's leavened hard tack. I think I can learn to eat hard tack.

Another thing to point out, is that biscuit literally means 'twice baked', as does biscotti. Now, biscotti is quite dry and hard as I think it really is baked twice, the biscuits we now eat aren't. I can imagine trying to eat some of my sister's biscuits... she wouldn't need to bake them the second time.
I remember reading about soldiers in wars gone past eating 'tinned biscuits'. In fact Lewis and Clarke explored the Louisiana purchase on Salt Pork and Biscuits.
I imagine hardtack is about the same as baking some Dakon (bison survival) flat bread after you pan cooked it, to dry it out.
This also looks to be about the same as Matzo bread (which you should be able to find in abundance around passover).
Now I am going to have to try to make some for a snack for at work. I am invisioning a multigrain hardtack with a good amount of honey or maybe maple syrup for flavour. Make it into something I would want to eat if I gotta work through lunch or I am running late to get somewhere.
Hrmm, mind wandering already... I an invisioning making a stack of 3-4 of these the right size to fit into a plastic sandwich bag. The middle one or two (depending on thickness) have the centers cut out so I can put 4 of those cafe-toast jelly packet inside. I now have a 'hardtack box' that has jelly stored safely inside.