Drobo 5N Not Recognizing New Disk Capacity


Drobo 5N owner here. I recently upgraded a drive from 3tb to a 6tb since I was low on space. I removed the indicated drive and replaced it with a 6tb, 24 hours later the data protection finished. I am still receiving low disk space alerts and still see the capacity has not changed. Now the drive slot below the one I replaced (3tb) is showing a yellow indication and wants replacement. I should have necessary space now.

I tried restarting the Drobo and I am on the latest firmware and dashboard.

I would appreciate any advice.

are they ALL 3tb drives? (or was that 3tb disk your largest one?)

if so then you need to replace TWO drives with large ones in order to gain additional space

try using the capacity calculator on the website to see.

the reason is that : you’ve given drobo 3tb more space (the “extra” 3tb at the end of that disk), but it has no where else to mirror that data to in order to protect it - if that makes sense? you should be able see it in dashboard as "reserved for expansion or something similar

another way to look at it: available space is always the sum of all of your drives… minus the largest one.

if you replaced the 6tb disk with an 8tb, or 10tb, or even (theoretical 20tb) your available space wont change until you replace a second drive with a larger one

Got it, sorry I derped for a second. I’ll install another 6tb disk. Does the 5n have any capacity limits?

Yes Drobo 5N currently has a internal limit of 16TB RAW Space…

I assume that your 5N setup currently If you have replaced with 2x 6TB drives…

3x 3TB and 2x 6TB = Total RAW Space 21TB
With Single Disk Redundancy = 15TB

That’s put you just within the 16TB limit (approx).

So in the future, if you swap another 3TB with a new 6TB …

2x 3TB and 3x 6TB = Total RAW Space 24TB
With Single Disk Redundancy = 18TB
*There will be about 2TB (Unallocated space) as the limit of 5N is 16TB.

Hopefully this can be addressed by next firmware update to upgrade the limit to say… 32TB.


Don, I believe the limit is 16 TiB (2^44 = 17.59 TB) and that it applies to the volume size, not to the raw size. I have five 5 TB drives so my raw capacity is rather more than that, but with dual disk redundancy my volume size is less than that.

Yes, what I mean is the internal EXT4 of the 5N is limited at 16TB…

so 5x 5TB = 25TB RAW
Dual Disk Redundancy = 3x 5TB = 15TB …

So the internal EXT 4 volume is still within this 16TB limit due to DDR.

But with SDR, you will have 20TB left of the 25TB total…
But 5N limit is 16TB volume… of 20TB - 16TB … you left with 4TB wasted not usable space (Unallocated).

I actually have about 13.6 TB usable, not 15 TB, which agrees pretty well with the capacity calculator. The capacity calculator suggests that if I was to switch to SDR I’d have 18.15 TB available. That exceeds the 16 TiB limit (notice the switch to binary from decimal) which is equal to 17.59 TB. The wastage would therefore be 18.15 - 17.59 = 0.56 TB, which is much less than the 4 TB that you suggest. Personally, I would never trust so much data to one disk’s worth of parity, especially given the extremely long rebuild times, so with DDR one could use five 6 TB drives and still come in below the 16 TiB limit (16.34 TB usable), as the capacity calculator confirms.

I think there’s a lot of confusion between the binary units of storage and the decimal units by which hard drives are labelled. When computers had amounts of memory measured in kibibytes and hard disk capacities measured in megabytes the discrepancies were small (1 kiB = 1.024 kB, a difference of 2.4%; 1 MiB = 1.024^2 = 1.0486 MB, a difference of 4.9%). But now that memory is typically gibibytes and hard disks terabytes the discrepancies are much larger (1 GiB = 1.024^3 = 1.0737 GB, a difference of 7.4%; 1 TiB = 1.024^4 =1.0995 TB, a difference of 10.0%).

I’m pretty sure that like me, Don is aware of the difference between TB and TiB; we tend to just be lazy when doing back-of-the-napkin type calculations and working out Drobo’s SDR capacity with five 3TB drives as 4 x 3 TB (when we are fully aware that the disk is only 2.7TiB, and its only marketing that’s pushed stated capacities to be in decimal “TB”, which windows and Linux won’t display that - OSX now does as I understand, and its probably sensible for most end users people to use the decimal counting too)

Most of the time it makes no difference unless you are trying to squeak in under the 5N’s single volume limit, in which case that 10% difference is “handy”.

So while there may be a place for a conversation about the minutiae of Drobo/EXT4/Windows/OSX counting mechanisms and the various limitations arising from those… I don’t think its helpful in someone else’s thread.

Except that here it does make a difference and it trips a lot of people up. Granted, some people are lazy, but many more are ignorant. A lot of people are annoyed about the 5N’s volume size limit, yourself included Chris. As things stand (with currently available disk capacities) it is simply not an issue to me. Since the OP asked the question, I feel it’s important to give an accurate reply and in that I endeavour to be neither lazy nor ignorant. There seems to be a consensus that loading up a 5N with the current maximum of five 6 TB drives is going to result in a lot of wastage. I wanted to point out that it is not the case. I apologise to dsmero for hijacking his thread, to Don for pointing out the obvious, and to anyone else who found my answer overly pedantic.

Edit: Apologies again, but my pedantry has got the better of me again! Contrary to what you say, Chris, hard drive capacities have in fact always been marketed honestly. I have a 1 TB HGST drive in my laptop that has a capacity of 1,000,204,886,016 bytes - if anything, they’re being slightly generous. By the time it’s partitioned, formatted and space is taken by the overhead that a modern filesystem imposes there is 999.35 GB of space left for me to use, but that isn’t HGST’s fault; they sell hard drives, not filesystems. The dishonesty is in the specification of the RAM capacity. I’m told my laptop has “8 GB of RAM”. It doesn’t. It has 8 GiB = 8,589,934,592 bytes, rather more than 8.5 GB, but since that works out in the user’s favour he complains about the hard drive manufacturers instead!