Why DPM can only take offline backup for some VMs under Hyper-V 2012R2 Host ? Need SCSI !!

I published a recent article on migrating/moving VMs from 2008R2 to 2012R2 Hyper-V host (you can check it at http://itcalls.blogspot.com/2013/11/how-to-migratemove-virtual-machines.html ), after successfully moving these VMs we faced another problem trying to backup them. We were using the latest DPM 2012R2 and we noticed that it added these VMs as offline only and for some reason it can't take online backup of them.

After some investigation we noticed an error on the hyper-v host with Event ID 10103 (check below image) which clearly mention that backup will fail because this VM doesn't have a SCSI controller.

So the solution was just to add a SCSI controller even if its connected to nothing, after that online/hot backup was taken smoothly without any problem.

So what was the problem ?

I discussed this issue with several Microsoft support personnel and It turned out that online or hot backup for a VM in 2012R2 Hyper-V host requires mounting a new VHD in the VM and then dismounting it later. Since only the SCSI controller can mount/support hot plugging of virtual disks, it became clear why we need this SCSI controller.

Old version of Hyper-V didn't work this way,  it required that the Hyper-V host mount the guest VHD as part of the backup process which is something Microsoft didn't like as it increase the surface of attack on this host.