Dropped drive leads to lost capacity with dual disk redundancy?

I have a Drobo 5N with dual drive redundancy.

This morning, the drobo reported that “a drive has been removed” (It was not removed, apparently, it just died…still spins, but drobo will not recognize it when re-inserted).

Fine, I can RMA the drive and buy a replacement before it gets here.

What I don’t understand is if I start with 5 4TB drives, giving me 10.9TB of capacity with 7.29 used for protection, why when the drive goes missing does it show my capacity at 8.76TB?

In theory, I should have 10.89TB until I lose 3 drives, no? (At which point my data is gone).

But even with 2 drives gone, my capacity shouldn’t change…

What am I missing here?

Oh, and also…should it be doing a relayout now that the drive has gone missing? It’s not.

Just telling me that I have 8.76TB capacity and that I’m at 100%.


hi richard, if you excuse a bit of on the spot workings out while i type here to see something…

if you have 5 drives of 4TB each
5x4TB = 20TB raw
20TB (-4TB -4TB for ddr) = 12TB actual
12TB (*0.9 for usual filesystem overheads) = approx 10.8TB available for data (maybe about 10.2 with performance margin)

the way i understand it, is that the 10TB+ of data, should be available/useable, if 1 drive fails, and also if a 2nd drive fails.

dashboards 7.29 is essentially the 8TB i mentioned above for ddr, but i think it would only be able to do a rebuild/relayout, if there was enough space available for all your data’s parity, and also the used % value i think needs to be about 84% or less.

if you have some time, would it be possible to grab a few screenshots of your dashboard tabs, and to post them up to imgur or similar with links here? (if you do though, please remember to rub out any sensitive/serial numbers before uploading to play safe etc)

Yes, that is my understanding as well.

Right…but shouldn’t that 10+TB of space be available still?

Sure…which tabs? It might be faster for you to tell me what info you want and me to relay it, since I’m working off a laptop that isn’t mine (running windows, which I barely use), so I don’t have my usual tools available for screenshots, editing them, etc…


Here are some screenshots of my dashboard…


ah thanks for the screenshots richard, i forgot to mention which tabs but you covered a good range so thanks for those…

i think it is calculating part of the values (based on what you would have if you started out with just 4 working drives), but still with ddr enabled. (for example with 4 drives of 4TB = 16 (-8TB ddr) = 8TB (*0.9) = ~7.2TB available)

im not 100% sure, and hopefully a dev or another user can chime in to help clarify, though it might be that it takes this value plus the free space that you had before the failure (if you can remember how much free or used space you had before?) but is still trying to reserve space for a rebuild to take place and why it shows all space is used.

i tried some tests over on the calculator here, though you are right it is still not 100% clear how things add up: