Undo Disks is a feature that saves changes to a virtual machine in a separate undo disk file in case you want to reverse the changes. The Undo Disks setting applies to all virtual hard disks attached to the virtual machine.

When you run a virtual machine that is using Undo Disks, any changes to a virtual hard disk are temporarily stored in an undo disk (.vud) file, rather than in the virtual hard disks attached to the virtual machine. As you continue to make changes to a virtual machine, those changes continue to grow in the undo disk. If you decide to either apply or discard the changes stored in an undo disk, that action applies to all changes stored in the undo disk—in other words, you cannot selectively apply or discard changes on an undo disk.

To turn the Undo Disks feature on or off
  1. If the virtual machine is running or hibernated, shut it down. For instructions, see Shut down a virtual machine manually.

  2. Open the Virtual Machines folder.

  3. Right-click the name of the virtual machine, and then click Settings.

  4. In the left pane of Windows Virtual PC Settings, click Undo Disks.

  5. In the right pane, click Enable Undo Disks, and then click OK.

After you turn on the Undo Disks feature for a virtual machine, an undo disk stores all the changes associated with that virtual machine from the time that you start using the feature. These changes can include data that you add or delete to a virtual machine, updates applied to the guest operating system or applications, or even the addition or removal of applications. All virtual hard disks attached to the virtual machine are unaffected by the changes unless you apply the changes. To delete the undo disk file, you discard all the changes. This effectively leaves the virtual hard disks intact and eliminates the possibility that the changes will be applied to the virtual hard disk.

Refer to the following steps to apply or delete an undo disk. You also can configure the virtual machine so you can choose to discard the undo disk when you close the virtual machine. For more information, see Close a virtual machine.


The undo disk file is stored in the same folder as the virtual machine configuration (.vmc) file.

To apply or delete an undo disk
  1. Follow the steps in the previous section to open the Enable Undo Disks setting.

  2. In the right pane, click either Apply changes or Discard changes, and then click OK.

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