With mine, you just buy an extra tray any time you want to have another mountable drive. Buy a tray, buy a drive, and mount it to the tray. Then, at least on mine, you swap the drives out by using a key to unlock the drawer, slide out one drive, slide another one in, and then re-lock it. It takes about a minute to swap drives. Even if you don't think you'll need the extra capacity now, you might down the road.
so what's the name, rank & serial number of this wonder ;^) ?