This discussion started here. Use Geizhals to find dual cards. The QNAP QXG-10G2T-107 seems to be the cheapest. But I can not guarantee that the drivers will work in Unraid. So order it with return option, so you can test it. The Intel X540-T2 could be a choice, too, but beware, many of them are fake (but seem to work without problems).   This is my research result regarding power consumption (for single cards):   X557-AT 3,4W TDP (not sure if total or only the contr