Well, I have WD MyBook world editon (white light) NAS with 2 X 1.5 tb in JBOD mode.Windows explorer reports 2.72 tb. Just like with 3tb disk in windows with ntfs formatting.The same disk in DNS-320L shows capacity of 2.68. So, I have lost 100 gb of space having 2 disks (3 + 4tb) installed. I have twonky media server installed but I doubt this could be the reason. Strange.