what deluge thinks is free space will depend on what you are pointing incomplete and completed folders to, for instance for me free space is 500GB, even though my array is much larger, this is because incomplete and completed are set to point at my cache disk and as you would expect, deluge is not aware of unraid or arrays so can only see free space on my cache drive.
so going back to your issue, see where incomplete and completed are saving to, is it to the array for both or are you downloading to your cache drive (assuming you have one).