I have a Drobo 5D and installed 2 Western Digital Red 6TB drives in it. The dashboard shows 5.42 of free space after formatting. When I use the capacity calculator on Drobo website and put 3 4TB drives in it a Drobo 5D(there is no 6TB single drives on website calculator so I used 3 4TB to get to 12TB) it shows available free space as 7.26TB. Any idea why I am losing almost 2TB of space when there is the same amount of raw data available installed in my Drobo 5D?
yes… you always loose your largest disk to parity (its an over simplification but basically true)
so with two 6tb disks, you lose one 6 tb disk (12 - 6 = 6)
with three 4 tb disks, you lose one 4 tb disk (12 - 4 = 8)
its because your data has to be protected somehow… if you only have two disks the only way to do it is to simply copy it, so everything you write to your drobo , one copy will go on one disk, one copy can go on the other disk, therefore you can only store 6tb
if you have three disks, rather than simply storing a copy , like when you only have two drives, (a mirror as it is called), drobo can be cleverer and store “parity”. so you can put 4tb of data on one disk, 4tb of data on another disk, and parity of that data on another disk. and if any one of those three drives fails, what is left on the other two is enough to recreate it
(again, thats over simplied, actually the data and parity is all jumbled up, but the concept is the same)[hr]
Disk A + B = C
…3 + 5 = 8
in the very very simple example above, we have three disks, A B and C
on A i have stored the number 3, on B i have stored the number 5, and on C i have stored the result of A +B, which is 8
if i were to remove any of those three disks… you could tell me what was missing by using the other two, lets pull out disk b, which has actual real data on it…
3 + ? = 8
easy, see
thats a massive oversimplification of how parity works, and how drobo protects your data, but the principle is the same, the maths is just much more difficult.
and it works for more disks too
A + B + C = D
3 + 4 + ? = 9
see 
Good explanation.
I just wish they would update the Calculator. It’s not like 5 and 6TB drives might not happen, they’re here, at least let people start planning and calculating…
Thanks for the explanation!
by the way, dont forget to calculate the overheads too 
once you have deducted your redunancy drives, times the leftover TB’s by 0.9 to get the actual total usable space for data