I can not explain my disk usage 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.
First of all, empty your trash, reboot &amp; try out disk utility again if you haven't already. If that doesn't do it though you'll have to dig a bit more.

Here's the terminal command for finding hidden files &amp; folders: defaults write com.apple.finder AppleShowAllFiles YES. Put sudo in front of that if it doesn't work - you'll have to enter your user password in this case. Once you've done this restart finder by clicking the apple icon in the menu bar and heading into 'Force Quit...'.

Now if you open Finder, select 'Go' &amp; 'Computer' from the menu bar, go into the drive in question. I would put this into 'list' view and make sure that the size option is on. For any folders that aren't showing their size, highlight them (hold command + click to select multiple) and then hit command + i to pull up the Get Info menu for them. You'll see that under the size field they're 'Calculating Size'. Just keep those windows up &amp; wait for them to finish - for the sizes you're dealing with this could take potentially hours. Add them up again &amp; see if the size is accounted for.

If there's still 500gb or so missing I would recommend to backup &amp; reinstall OS X. You can select only exactly what you want to using Time Machine, see the size before you even finish backing up, and then restore.