I know topic is old but some people (like me) still buying this mainboard for home NASes. I tried ECC memory on both slots and it machine worked fine. Probably its because its unregistered/unbuffered ECC and it has been recognised as nonECC. I also tried mixed setup with 4GB of Kingstone non-ECC and worked fine as well.
HOWEVER! I tried similar mixed setup on my computer with Asroch z68 pro3 (2x4GB nonECC + 2x2GB ECC) and after 15 mins pc crashed and had a bootloop. So even if Asus much more tolerates mixing or ECC itself i would STRONGLY reccomend make a memtest for at least 24h or more.
