2x16TB Partitions mounted since I replaced one failed hard drive

This is freaking me out:

I replaced a failed WD 6TB drive with a new one, Drobo Dashboard kept asking me to format the newly inserted drive as it couldn’t read it (a dialogue popped up that I didn’t remember seeing before when replacing drives.

After the ‘format’ a second unit popped up on my desktop with supposedly 16TB space (which is technically not possible due to DUAL DISK REDUNDANCY engaged) See screenshot:


And here within Drobo Dashboard:


Can anyone enlighten me what’s going on and what to do to?

How to

A) lose that new ‘bogus’ partition
B) NOT lose any data or having to worry about what’s going on
C) explain what happened to prevent it from happening next time

Yes, I do have a current back up but I cannot afford to reformat the system and then restore it due to deadlines for projects.

Mac OS X Yosemite 10.10.5
5 x WD Red 6TB,
1 x WD Red 8TB

Thanks guys,


Was the 8TB drive the replacement for the failed 6TB drive? If so, it seems to have bumped your capacity past the existing 16TB volume limit, so Drobo created a second 16TB volume for you to format and use. That is called “thin-provisioning” and is a normal Drobo function.


As DragonLord said - everything is working perfectly.

If you have less than 16TB of available space - you get one 16Tb partition. Obviously since you have less space than that, it is what is called “thin provisioned” - so its lying about howmuch space it has - this means that you can easily add space after without having to reformat the filesystem…

You said that drobo asked you to format the newly inserted disk. this is not true since drobo never allows you any access to the individual disks.

what actually happened was:

  1. you inserted a new disk
  2. this increased your usable capacity to over 16TB.
  3. since you can only use 16TB with one volume… and you now have more than that, Drobo automatically created a second volume for you to be able to use all of your available space (so is if you have 18TB of usable space - you can put 18TB of data on there now, spread however you like across your two thin provisioned 16TB volumes.)
  4. there is no way to delete it, and you would not want to as that would leave you with space on the drobo which you cannot use.

it is interesting that when you had, say 14Tb of space, you were OK with having a 16Tb volume… (despite that fact it woudlnt really fit)

but now you have 18Tb of space, the fact that you have two 16 TB partitions is “freaking you out” because they wont fit…


see the logic?

I hope i explained that ok…

hi guys can i just check something please…
did louis originally have 4 x 6TB drives + an 8TB drive before any drive swaps?
if so, would that not mean that the same volumes would already both exist (since it was just a failed 6TB being swapped out and replaced with a replacement 6TB?)

the first post mentioned 5x6TB drives and an 8TB drive so i wasnt sure if it was actually one of the 5x6TB drives that was actually replaced (and upgraded) with a larger 8TB?

[quote=“AzDragonLord, post:2, topic:142650”]
Was the 8TB drive the replacement for the failed 6TB drive? If so, it seems to

Thanks for taking the first step in trying to explain what has happened @AzDragonLord! Much appreciated!

I did, yes, thank you. Please see below though.

Paul is right!

Sorry for the confusion, but the mind boggling part is that the system HAD THE SAME CAPACITY before the swap of the failed 6TB drive with a NEW 6TB drive. And there was NO EXTRA partition.

While I now at least grasp the concept of thin-provisioning in this stage I still have no idea what that actually means for practical work and more importantly back up scenarios:

  1. Can I use the second partition to have faster access to files and use it as a ‘clean partition’?

  2. Do I have to back them up separately?

  3. If I start splitting up data between the two 16TB partitions how would I know how much I have left in total as the first (original) one is showing me:

Partition A = 8.35TB available for use
Partition B = 16TB available for use

If I continue to use up more space how will I be able to know what my actual total would be?

  1. What’s a clean solution to get rid of this ‘mess’ (sorry, I just don’t enjoy this whole splitting thing especially as it seems to have happened randomly after nothing in the setup has changed capacity wise and I don’t want to have to guess and wonder all the time about capacity or reliabilty issues…)

  2. What happens if 2 hard drives fail? Will the maxed out second 16TB B partition be able to be saved??!?! I just don’t understand how it would be possible to fill up 32TB now and have that saved on any combination of potentially 3 x 6TB drives if two should fail. (Dual Disk Redundancy enabled)

Clearly I’m either very confused or something’s off.

I would really appreciate another take on this from you guys! Thank you in advance!


This might help…

following on from dragons links, i was just thinking about some possible theories…
(i dont know if they are true but the theory seems a bit plausible) :slight_smile:

  • with blank cds, some manufacturers included more track length which was usable in creating a cd that could hold more data. (for example there were the usual 74min blanks, and then some 80min blanks, but some 80 min blanks actually let you store about 81mins worth of data. then some were 99min cds but each came with certain options or restrictions such as which player or reader would recognise them fully)…

… with this in mind… is it possible that some makes of hard drives are simply called xTB drives because they have at least xTB of space, but that maybe the have a little bit more? (if so, is that extra space essentially balanced or cancelled out by the SDR/DDR process?)

  • also, is it possible that the original drive had some bad sectors that were unusable (but that the drive lasted as long as it did without any more bad sectors appearing) but that this slightly reduced capacity meant that the final total usable space for data was still below the 16TB mark (and it was also balanced out via sdr/ddr?).

theoretical ramblings aside though, here are a few quick thoughts on your recent numbered questions loius: (and where possible, the thoughts have about 1 word by contrast…at the start) :smiley:

  1. Possibly, but you would still need to manually keep track of how much data you have on the whole drobo so that you do not risk overfilling. :slight_smile:

(i say possibly because while each volume is actually being served via the same hardware unit, volumes or drives on most devices generally tend to slow down as they become full, especially near the end, but you probably wont reach, and probably wont want to reach that level of “fullness” with your current setup).

  1. probably :slight_smile:
    i think it depends on how you do your backups, and what tools you use.

for example, regardless of the number of volumes i have, (and seeing as i actually require, and prefer, a maximum volume size of 2TB, i have a fair few across my drobos), i still manually select which file/folders to backup, from which volume at a time, and which destination folder to use. (depending on what is being backed up, i also use a very good windows tool which is part of the syncback range, which allows me to set-up backup profiles, but rather than simply backing up each virtual volume at the volume level, (unlike when you need a full single hard drive image via acronis or similar), the profiles are set to include all files or folders within the defined root, but with specific exclusions on certain special folders, (or some folders that i am not wanting to include in that particular backup profile).

  1. Dashboard is your friend :slight_smile:
    as far as i know, windows will report the correct amount of Used space, even if the max volume size with free space is higher. for example you could select all files, and do a properties to get the used space (but ideally would use the largest reported sizing in properties dialogue, and would be able to view all potentially hidden files or system folders etc)

The main volume (drive letter) window could give properties too, but i think the best way is to see how much free space is left, via drobo dashboard.

  1. embrace - i would try to embrace this particular change (partly because it is not actually that bad and can have some benefits as i might have mentioned on some other related posts, but also mainly because you are so close to the 2nd volume borderline anyway, that you will most likely need to have another volume if you significantly expand any more)

another user was able to use a windows server and another 3rd party tool (which i dont think is free) to essentially merge multiple volumes across multiple drobo units, into 1 large pool - but i honestly dont think that will benefit you at leasst not at this stage, and can also add a lot of other variables in the mix.

  1. relax :slight_smile: (i think as long as you follow step1 and use dashboard for your remaining total space monitoring, then DDR should be able to still cover you in terms of 2 drives)
    (i was going to say “frankie goes to hollywood” as the 1st word :smiley: