The main issue is you don't understand how manufacturers list their drive capacities.
They use 1000, 1000000, 1000000000, and OSes use 1024 instead.
There really is nothing on it, as the space never existed in the first place.
NTFS according to that screenie, Underlying on the Freenas box will be UFS.
I withdraw my first answer, and that picture leaves me rather bewildered.
I'll have a check through some things, seems an exceptionally large amount of space 'used'.
There isn't another partition of that size that can't be read and thus is classed as used space?
Its not using some form of cache is it?
doesnt freenas default having a cache/swap space just like linux?
You could also modify the "du -h" command to be "du -chs *" which gives a breakdown of which folders contain the usage, with a total at the end.
You can use LVM to achieve 'one' storage area but you're still at risk of losing data if one of the drives fail.