Krypton hard drive

If it's the same chassis as the Krypton I purchased in April of 2014, there is only room for 2 drives total.  Mine has 3 storage because at the time I ordered it, DS offered it with an mSATA boot drive, so I still had two bays available, one with the data drive I ordered for the initial purchase and a third one I added later.  As for compatibility, any laptop platter drive or standard 2.5" SSD should fit.

I'm not quite sure what you mean.  When I bought mine there were two bays available only, one which contained my data drive and the other one empty, to which I added my 850 EVO later on. DS stopped offering the mSATA option that I used for my primary drive not long after I got mine, so at that point, you were only ever going to have two drives unless you added an mSATA on your own after the fact.  I never heard an explanation as to why they stopped offering the mSATA so quickly, and have long wondered about that.

Drives are always going to be available cheaper as time passes  with newer models coming out, and especially if you add them after the fact and don't purchase as part of the original config.  You'll see that from any laptop vendor.  I still see other companies charging $150 more for a 128GB SSD instead of a platter drive, despite the fact that you can likely buy the same model from NewEgg or Amazon for less than half that.

As for the guts of my Krypton, the two drive bays are in the same compartment.  One is beneath the optical drive, so you have to remove that (very easily) to access the bay and add the second drive.  As you have both bays filled, you're likely going to need to replace one of them with something larger.  As far as I know, unless you add an mSATA to a Krypton, you're not going to be able to go for three drives.