I have a RAID 5 solution with 4 disks of 2TB. That gives me 6TB disk space. Now Disk Utility tells me that my actual usage is 3,5TB and I have 2.5TB free space. But when I add up the disk usage of the individual folders then this does add up to 2TB. So there is a gap of 1.5TB.
I have deleted a very large folder recently. Could that explain it ? That disk utility is not reporting the actual situation ?
EDIT: I have downloaded OmniDiskSweeper and this app gives this list of my disk group:
So my folders add up to 2.0TB. Here is the disk info that my MAC gives me:
As far as I can tell I see a difference of 1.5TB. I am no techie so I cannot explain this.