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Q. If I'm using a differencing virtual hard disk (VHD) and the parent VHD is corrupted, have I lost all the data in the differencing disk?

A. Unfortunately, yes. Remember that a differencing disk has a child relationship to another VHD, and only new or modified data is written to the differencing VHD file. This means if the parent VHD is no longer available, the information in the differencing disk is incomplete and wouldn't be usable. The parent-child relationship is shown below.

As you can see, data can be read from the differencing disk, but reading from the differencing disk may actually read the data from the parent disk, if that is where the data was originally and it hasn't changed. When you make a write, the data that has changed gets written to the differencing disk. This means that if you lose the parent, the only data in the child would be the delta blocks, such as the seven characters in the example shown. These bits aren't useful.

Related Reading:
  • Q. What happens when I take a Hyper-V virtual machine (VM) snapshot?
  • Q. What is Quick Storage Migration (QSM)?
  • Q. Why can't I take a Hyper-V snapshot of a virtual machine (VM) that has a pass-through disk configured?
  • Q. I'm deleting a Hyper-V virtual machine (VM) that had snapshots. Why is the VM delete taking so long?

  • Check out hundreds more useful Q&As like this in John Savill's FAQ for Windows. Also, watch instructional videos made by John at
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