Drobo FS reports critically low on capacity but it should not be

My Drobo FS is reporting that it is critically low on capacity, but it’s not. Or it shouldn’t be.
It is populated with 5 healthy drives of 3TB each, it’s configured for dual disk redundancy and should thus have more than 8TB of usable space and the data I’ve put on the Drobo is only 3.9 TB.

Quick backstory:

  1. The Drobo FS has been quite stable for some time, though I’ve had to replace drives over the years. Mostly WD NAS RED drives now. When this started it had four 3TB drives and one 2TB drive.
  2. Oct 20: unplanned power outage, after which Drobo Dashboard reported “mount failed.” I proceeded with a repair operation (initiated through Drobo Dashboard) which took only about one day, after which I rebooted (again through Drobo Dashboard) and it was then able to mount everything but it showed a red light on the 2TB drive saying the system is critically low on space and the drive must be replaced with a larger one.
  3. I first replaced the 2TB drive with another 2TB drive I had on hand and after the data protection operation completed, it complained of the same problem (critically low on space)
  4. I replaced the 2TB drive with a 3TB drive, and after about 1.25 days of rebuilding it again says critically low on space and I should replace the top drive (3TB).

I don’t know how to interpret all of the diagnostic space readings:

TotalCapProtected: 8908807864320
UsedCapProtected: 3875358380032
FreeCapProtected: 0
TotalCapUnprotected: 0
UsedCapOS: 8908807864320
Unallocated: 0
UseUnprotectedCap: 0
AvailableForData: 8908807864320
AvailableForData_SDR: 11981860061976
AvailableForData_DDR: 8908807864320
UsedForProtection: 6078773146194
ReservedForExpansion: 0
Overhead: 15383899566
TotalPhysicalCapacity: 15002964910080
UnallocatedCapacity: 0
TotalCapacityPT: 0
CapacityCache: 0

But TotalCapProtected, UsedCapProtected, and TotalPhysicalCapacity looks correct.

The device seems otherwise happy, and I can mount the shares but have avoided doing so in case I might cause some corruption.

Has anybody seen this behavior, can you recommend what I should do next to resolve it?


First things first.
Please make sure you have an external copy of your important data.

Can you post a screen shot of both your capacity pie-diagram and the usage ribbon?

Thankfully I’ve had an automatic backup running weekly to another drive, and Backblaze, so loss of data is not a concern but would be a major inconvenience!

Requested images attached, thanks for taking the time!


Thanks for the images.
Great to know you have an external backup!

The screenshots don’t give me any insight unfortunately either. I take your word on the data quantum you have stored, over the info displayed on the Dashboard which has alerted you for further upgrade, however in this case, it would require 2x HDD as adding only 1 larger HDD will mean that its data delta will specifically be reserved for expansion with no benefit - hence the need for 2 HDDs - if your volume permits it.

I’m left with the suggestion that you should reformat your Drobo volume completely and repopulate it with your data again with your backup. As you mentioned you have used if for years, I have found that my Drobo directory can get bloated/corrupted over time and responds much better after a cleanup every quarter or so. A reformat would certainly achieve that.

Thanks for the suggestion, I’ve done a “reset” via Drobo dashboard and the device is reporting capacity properly now. A few days into the long process of copying everything to Drobo again from backups.

Great to hear. Thanks also for updating your status.