Missing Storage Readynas 516

If a disk is labeled as 4TB, it only has roughly 3.6 TiB. (read wikipedia here for the difference https://en.wikipedia.org/wiki/Tebibyte). And the nas shows you the lower on its admin webpage

Furthermore running your system in raid5 you are "loosing" 1 complete disk to have redundancy insurance. If one disk breaks you would be able to replace it without loosing everything.

So, your 6x4TB disks in raid5 gets us 5x3.6TiB, which is roughly 18 TiB diskspace overall for your system configuration.

As you are reporting only 14.5TiB in your system overall, there might be the problem, that not all disks are recognized to their full capacity.

You could do the following to find out what capacity the nas recognizes for each of your disks. On the admin webpage go to System/Volumes and hover your mouse for a few seconds over each of the blocks representing a disk. In the 7th row of each displayed text-box you will the see capacity for the concerned disk. Under normal circumstances I would expect for all 6 something like 3.6.