Windows Server 2012/2012R2 – When Physical disks are attached to a virtual machine, Hyper-V Replica will have to be disabled

Windows Server 2012/2012R2 – When Physical disks are attached to a virtual machine, Hyper-V Replica will have to be disabled

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Hello Everyone,

Many of you have come across issues with respect to Hyper-V replication in Windows Server 2012 and 2012R2.

In this blog, we are going to address an occurrence that you might run into when enabling Hyper-V Replica. In Replica, you cannot enable a Virtual Machine with a physical disk attached to it (iSCSI, physical disk, and Synthetic Fibre Channel) This is by design. Below is a screenshot of the error message you will receive.




Hyper-V Replica works by tracking all writes that go to the virtual hard disks attached to the VM. The write commands are then saved to a log file and shipped to the Replica server. From there, it is applied on the replica virtual machine.

The logic used for tracking these writes is implemented in a driver called VHDMP.SYS that runs on the host machine.

This driver only tracks writes that go to VHD files; it does not track the writes that go to any physical disk that are attached to the virtual machine.

Similarly – any physical disks that are connected within a VM are invisible to the host machine and hence VHDMP.SYS does not see these disks.

As a result, the physical disks that are connected cannot be replicated to the destination.

Work Around:

Replication can be done before any physical disks are connected to the Virtual Machine. After replication is completed, add the physical disks back.

In the case of iSCSI attached physical disk:

  • Remove the targets and stop the iSCSI service.
  • Replicate the Virtual Machines and then after replication is done, start the service and then add the targets. This has to be done anytime you wish to replicate the VM over as we will again detect any physical disk upon the next replica cycle (5 minutes in Server 2012 and configurable in Server 2012 R2).

Hope this is of help to all of you.

For more information about Hyper-V Replica, see:

Deploy Hyper-V Replica

Configuring Hyper-V Replica in Windows Server 2012

More to come soon.

Suganya Natarajan
Support Engineer
Windows Core Team

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  • WHy not simply show a warning-dialog instead, asking if it's okay to proceed neverthess without the detected disks that cannot be replicated or abort...?

    This is unnecessary complicated IMO

  • Hi,

    it is way easier if you just disable backup in the integration services of your virtual machine, setup replication and enable it again.

    But as Bart van de Beek just add an option to ignore attached disks as in many cases there is an cause why the attached disk is not a vhd.

    Best wishes,


  • Yes, please let us replicate even when disks are attached that can't be replicated. I'd like to replicate VMs between locations that share iSCSI storage that is used from within the guests, but the current behavior effectively prevents me from using replication at all. And that although not-replicating the iSCSI disks is exactly what I want / need.

    Protecting the ignorant from a false sense of security is one thing, preventing use in valid scenarios another.

  • Hi John, I've been struggling with our new Hyper-V Replica infrastructure, a site HV-cluster to another site HV-cluster- the issue is every time I tried replicating a VM from a hyper-V cluster to another via a broker, the virtualized host got frozen and I had to hard reboot it. The same problem occurred when I tried testing it using PS. To your understanding, we have HP enclosures for both sites, same HW/Specs, using Blades; SAS for host's storage, CSV 's are used in each cluster. The platform for all nodes are W2012 Data Center. All VM HDs are sitting in CSV, no physical disks attached, still the problem, and I got stuck at it Appreciate if you can advise what's the cause, hence the solution. You can mail me at BTW, my name is Hairauva,