First screenshot shows what happened when I tried to launch the render of my 3x5 (x3 layers). I asked for the layers to be output separately. Second screenshot shows what happened when I quit AP, re-ran it and loaded the .pano file. Only 1.51TB needed now but this is still way over what is needed.
Loaded same images into AP 2.6.4 and asked for the same render - it went ahead without any complaints.