Virtual PC Differencing Disks

This post is for me as much as it is for others.   Maybe by writing it, I’ll finally remember how.

Because I do it so infrequently, I can’t remember how to get set up Virtual PC to use a differencing disk without getting the error:

“The startup disk could not be written to.   The virtual hard disk may be marked read only.   The virtual hard disk has been mounted read-only.   Most operating systems cannot boot with a read-only startup disk.”

The answer is easy and it makes sense if you think about it.   The short answer is to not have your parent hard disk referenced in your Virtual PC settings as a hard disk.   Since the differencing disk is linked to the parent, the differencing disk should be set as your Virtual PC primary hard drive.   Virtual PC uses the link between the two to determine which disk actually has the OS installed on it.

Here’s the detailed steps:

  1. If you haven’t already, create your parent hard disk using the Virtual Disk Wizard and install your OS to the point you are happy with it as a base.
  2. Shut down the guest OS.
  3. Mark the .vhd that is your parent disk as read only via the host OS.
  4. Create your differencing disk using the Virtual Disk Wizard and selecting the disk from Step 1 as the parent.
  5. In the Virtual PC settings, set Hard Disk 1 as the differencing disk created in Step 4.
  6. Make sure you have no references to the parent disk as any of the hard disks in Virtual PC.
  7. Boot and enjoy your Virtual PC with a differencing disk.

The key is step 6 – you don’t want to reference the parent disk as a hard disk in the VPC settings.   Because a differencing disk has a parent disk specified when you create it, Virtual PC knows where to find it when you set the differencing disk as the main disk for your VPC.

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One Response to Virtual PC Differencing Disks

  1. Simon pieman says:

    Thank you so much. It never occurred to me to reference just the differencing disk! You have saved me a lot of time and the world is better place after your post

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