High water marks are based on the largest drive, in your case 8TB. Once there's less than 1TB available on it, then it'll start filling disk 2,3,4 & 5 until there's 500GB available, then start over again until 2,3,4,5 have 250GB, then 2,3,4,5 until 125 GB, then will start putting files on 1,2,3,4,5 once there's 62.5GB. Assuming of course that disk 5 isn't excluded in the global share settings (Settings tab)