Confused about "Protected Space"

I have had my Drobo 5D for some time, and never had any issues. I have: slot 1 - 3TB drive
slot 2 - 3TB drive
slot 3 - 3TB drive
slot 4 - 4TB drive
slot 5 - empty
msata ssd 120GB
If it click on Drobo Dashboard - “Capacity” I get:
8.12TB available for data
931 GB reserved for expansion
2.77 TB used for protection
14.32 GB overhead
When I click on “Volume” I get:
7.58 TB used
16 TB Maximum capacity

Windows shows the Drobo drive has 8.41TB free space.

So, my question is, I get a yellow warning: "Drobo is running low on protected space. Insert new drive in empty slot.

I’m OK with doing this, and have a new drive on order but why is that needed if I have 8TB available with my current configuration?


After I posted this, I clicked on Drobo Dashboard “list” icon. It states:
capacity - 8.12 TB
used - 7.58TB
free - 559GB
I’m guessing this is the reason it needs another drive…???

Welcome to thin-provisioning, which gives the appearance of having more physical resources than are actually available.

The usable space Windows “sees” is your 16TB volume (your 5D is capable of a maximum 32TB volume which you select during initial setup). Your “actual” capacity is determined by adding your drives and subtracting the largest one. Thin-provisioning is what allows you to increase capacity by adding larger drives without re-formatting and starting over with each change.

hi just to follow on with what dragon said above,
if you also use DDR in future, then it will take up the capacity of 2 of your largest drives (since its then allowing for up to 2 drive failures)