Tip of the Day: Configuring Disk Mirroring for Windows Server 2012

Today’s Tip…

With the release of Storage Spaces in Server 2012, a common question is how customers can use Windows to provide resiliency for the OS volume. In Server 2012, Spaces does not support the OS volume, but customers don’t necessarily want to use a hardware RAID card or a 3rd party produce only for the OS volume. 

This new whitepaper explains step-by-step how to set up a Dynamic Disk Mirror of the operating system volume, allowing customers to avoid the expense of using a hardware RAID card:

Configuring Disk Mirroring for Windows Server 2012

Comments (26)
  1. colby wilksch says:

    hello i own a acer altos R520 series server running win server 2012 r2 data center with a raid key(home use) and during the booting process i have entered hard drive management (not sure what exact name is) and attempted to mirror 2 hard drives halfway through the process the power cut out after rebooting i discoverer-ed the mirror had failed and the hard drives became corrupted only had the two hard drives connected is there anyway to reverse the process/ recover the hard drives ?

  2. colby wilksch says:

    i own a acer altos R520 series with 2 hdd’s running win server 2012 and have attempted to mirror one hdd to another but my power source cut out when i rebooted i sadly found they had become corrupted ( incomplete mirroring) is it in anyway possible to restore my hard drives ? (it was done during the hard drive manager during booting)

  3. Matt says:

    This is insane. Has 2016 improved this process any? I can’t believe this has only gotten worse since 2008, almost 10 years ago.

  4. John Smith says:

    I can’t believe how many pages this is. What year is it again? Surely it can’t be this hard to setup a software mirror between OS disks.

    1. Bill says:

      This has to be a joke. Considering how remarkably easy it is to create a raid on CentOS I would have thought Windows server 2012 and 2016 would have implemented this!

    2. jdz.tsg says:

      It’s not that hard.

      For some reason, they are overlooking the obvious configuration that has been used successfully for years on Windows 7/8/10 Pro and Server 2008/2012/2016 BIOS-based systems. Mirror the “System Reserved” partition as well as the Windows partition. Then when you update the BCD, your are updating both disks at the same time. No matter which disk the computer starts from, BOTH menu choices are available.

      This is especially helpful when the Window OS partition on the Primary becomes unbootable for some reason. Countless times I have been saved by just selecting the Secondary OS to get the system up and running and then reestablishing the mirror for the Windows partition while the system is online.

      As Mr. Webb stated, “At least with drive mirroring I can choose a second boot drive from the boot list and get the server back online while I find another drive to replace the dead one with.”

      There is an open thread right now at (https://social.technet.microsoft.com/Forums/en-US/4d6b426e-5aab-4f82-835e-d4c141e1c01f/windows-10-mirror-disk-not-showing-in-boot-menu?forum=win10itprosetup) where a client system is NOT automatically creating the BCD entry for the secondary mirrored disk and displaying an error that it cannot update the Boot Configuration Store.

      There is also an open support call with MS about the same subject. So far, the forums and MS Support Engineers are pointing to this document as an answer even though it isn’t since the document assumes that the “System Reserved” partitions are not mirrored.

      Further, it does not explain how to manually create a new entry for the Windows partition on the Secondary Disk if, for some reason, Windows can’t/won’t create it. On a working system with mirrored “System Reserved” and “Windows” partitions (where Windows has automatically updated the BCD when the mirror was established), the BCD entries for both disks appear identical. They both point to “partition=C:” for device and osdevice. They are obviously different as the one with a description ending in “- secondary plex” will boot from the second disk. But I cannot find instructions to manually create this entry.

      If anyone has answers or suggestions, they would be appreciated. Thanks.

      1. jdz.tsg says:

        This issue is resolved. The solution is 1) Boot to the recovery environment. 2) Break the mirror using DiskPart. 3) Rebuild the BCD using BootRec /RebuildBCD. 4) Boot back into Windows on the Primary Disk. 5) Delete the partitions from the Secondary Disk. 6) Remove the old entry for the Secondary Disk from the BCD using BCDEdit. 7) Remirror the Primary back onto the Secondary (System Reserved and Windows OS partitions).

  5. Ezra Small says:

    hello everyone i have a question on how best to perform server mirroring over the internet, like i have two sites in different locations but i want to implement server mirroring how possible do i do that?

    1. Mikhail says:

      read about Hyper-V replica

  6. Paul Webb says:

    For those asking “Why not just use Hardware RAID instead” here’s a perspective from someone that’s been doing infrastructure for 20 years:

    One driving factor can be whether you have the space for a hardware RAID card. For example, the PERC H[xxx] cards from Dell are nice, but they take up a PCIx slot on the mainboard. If you’re using a Dell PowerEdge R210 or something similar, you have exactly one PCIx slot available, and frequently we use these servers for things like a standalone physical vCenter ESX head end for our vSphere cluster, Firewalls, Phone Servers, CCTV Servers, etc. In many cases for one of those functions, I need that one PCIx slot on the board for the intended purpose — like it needs to be used for a T1/E1/PRI board in the telephony server solution. So, there’s one aspect.

    Another aspect is the whether or not to use the On-board RAID controller in the mainboard. I typically advise against this. Onboard RAID controllers (or Software RAID controllers) are typically a joke and you can lose your RAID array simply because the software RAID decided to bail on you and lose your configuration on a reboot. And the import configuration function can be a joke, too. I’ve had more arrays fail because of this than anything else. Also, the whole reason behind RAID is to allow you to remain up while you get replacement hardware. Well, with software RAID most often I find that the server reboots itself and then hangs at the screen saying “I lost a hard drive!” or “I can’t find the configuration!” which means my server is DOWN.

    So, if Hardware RAID isn’t viable and Software RAID is worthless, what’s NOT worthless? Drive mirroring. At least with drive mirroring I can choose a second boot drive from the boot list and get the server back online while I find another drive to replace the dead one with.

    Hope this makes sense and explains my Disk Mirroring might be preferred over Software RAID.

  7. Mike says:

    Whoever came up with this elaborate process for creating a software mirror had to be enjoying the new lax medical marijuana laws. It is like 2 pages that can be summarized by the original windows 3.1 documentation. At the command line type Format C: . The computer will respond, “Are you sure y/n”. Press y and then Enter. Who the heck is going to create, change, and manipulate active partitions on their system disk that took literally hours to create.

  8. chard says:


    I am following the guide and I am doing it for Windows Server 2012 R2.
    I am encountering error below:
    REAgentC.exe /enable
    Operation Failed 70
    An error has occurred

    Can someone help.

    1. Mobius120170 says:

      I have the same error 70 where I cannot enable the REAGENTC after setting the image? Did anyone sort this out following this document?

  9. TM King says:

    I am attempting to mirror a NON-OS partition drive with an exact physical copy- The drives are 1.5TB NVMe drives. After completing the steps in the document above, I receive the following error:

    Extent Disk1-01 on disk {74780285-5968-11e6-80c0-0000ba202eb7} that is part of the fault-tolerant volume D: is no longer accessible.

    Google is no help. TechNet is no help. Trying to find any relevant info based on the Windows Event ID is an exercise in futility.

    The drives are physically identical. They have not been formatted or manipulated in any way.

    After the formatting process (which is the last part of the “New Mirror” Wizard, the drives immediately go to “Failed Redundancy”, and nothing I can do (reinitialize, reactivate) restores the drives.

    Any ideas?

  10. Aleksandrs Jurcevs says:

    I can’t find “Entry: Description” and “Device Options: Ramdisk Options” in BCDStore. They are not there. On Win Server 2012 R2, and R1 too. What to do? What is it, and hoe it work?

    1. Are you attempting to update the BCD Store on the secondary disk? If so, there are several examples listing Ramdisk Options if you search for images using bcdedit /store /enum all server 2012. Here is an example. http://www.deploymentresearch.com/Portals/0/Blog/Files/1/120/Windows-Live-Writer-Speeding-up-the-1E-PXE-Lite-boot-process_A0D2-NomadPXE2_thumb_1.png

      1. tomas says:

        hw is talking about R2, so do I: no ramdisk options …

  11. You’d be better off just using hardware RAID for your OS drive. Storage Spaces will not allow you to do software RAID on the OS drive. So the only thing you really can use is dynamic disks. However, this is an older technology that is in the process of being deprecated.


  12. Mandla says:

    I just did this as follows on windows 2012 r2
    1. Add 2nd drive to system and boot up
    2. Convert primary C: (1st disk) to Dynamic disk via “Disk Management”
    2.1 If you get an error about insufficient space simply shrink you partition on primary c: (right-click “Shrink volume”)
    3. Delete any partitions you may have on 2nd drive
    4. Right click and “Add mirror” starting with System reserved partition
    5. Right click and “Add mirror” now on C:

    That’s it.

  13. MaxO says:

    For the BIOS set-up in the referred document, I’m trying to understand the reason for step 4 and 5 of “To Update the BCD store on the primary disk”. Why would you delete the entries for the “Resume from Hibernate” and “Windows Boot Loader” from the primary disk?

    The same entries (secondary disk) are also deleted in step 4 of the “To update the BCD store for the secondary disk”, but subsequently restored/updated in step 3 of “To update the secondary plex entries on the secondary disk”.

    Does this mean that I can boot from the secondary disk but not the primary disk? Does anyone have followed the steps (for BIOS) and have successfully booted from the primary and secondary disk by alternately removing one disk?

    Unless I’m missing a step, with those entries deleted (mainly the Windows Boot Loader) in the primary disk, the primary disk may not boot?

    I would appreciate if you can share your experience. Thanks.


  14. George P says:

    My conclusion: By the time any sysadmin implements this (assuming no errors), the labor cost would have far exceeded purchasing an actual RAID. I’m sure there probably are times when this will be optimal but I do not see it working for small and medium sized companies. Am I wrong here?

  15. T_Meyer says:

    Follow-up: confirmed; the system drive mirror itself works fine. The "System Reserved" partition will need be mirrored manually but updates are rare.

    I’m using this for a lab server with SSDs and without a RAID controller, allowing TRIM (thanks AHCI mode) to continue to work in "RAID 1". Data volumes will use Storage Spaces.

    1. Mandla says:

      I just did this as follows on windows 2012 r2
      1. Add 2nd drive to system and boot up
      2. Convert primary C: (1st disk) to Dynamic disk via “Disk Management”
      2.1 If you get an error about insufficient space simply shrink you partition on primary c: (right-click “Shrink volume”)
      3. Delete any partitions you may have on 2nd drive
      4. Right click and “Add mirror” starting with System reserved partition
      5. Right click and “Add mirror” now on C:

      That’s it.

  16. T_Meyer says:

    I think they’re saying the "System Reserved" partition isn’t mirrored. The situations where that would be updated by a patch are pretty rare, though yeah, it isn’t the best setup. Obviously the software mirroring is kind of a hack to begin with, so I’m
    not surprised they neglect this area.

  17. Doug says:

    If it’s true, it is a joke. They need to elaborate on this statement.

  18. Is it a joke? says:


    I just followed this link, and in this document they say:

    "Windows does not automatically service the contents of the secondary system partition. If you install any operating system updates or service packs, be sure to manually update the secondary system partition (the mirror)…"

    Folks, this is a joke, isn’t it? I setup a mirror – and have to copy certain files manually to keep the mirror in sync?


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