I checked the Asus website and realized there is a difference between the m.2 mount on Z10PE-D16 WS and Z10PE-D8 WS. The m.2 mount on Z10PE-D8 WS is pci-e 3.0 X4 while on Z10PE-D16 WS, it is only pci-e 2.0 X2. That explains it now. I actually didn't need all the memory slots on Z10PE-D16 WS ( 2 x 32GB LRDIMM on order). I chose Z10PE-D16 WS simply because it was less expensive! I sometimes think the Z10PE-D8 WS was perhaps a better buy. I only prefer the configuration of the mofset heatsinks on Z10PE-D16 WS.