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I have been trying to install a training pack for Microsoft BizTalk course 2934a. I have downloaded multiple VHDs and followed the instructions (I think!) for installation. However, on attempting to start one of the VHDs from the Virtual PC 2007 console I get the message "The parent virtual hard disk has been deleted or has been moved from its previous location. Please select the new location of the parent virtual hard disk". At this point it's telling me that it's looking for Base05D.vhd. I already have a activated copy of this huge base disk so I browse to the location and select it. The I get the message "The virtual hard disk's parent appears to have been modified without using the differencing virtual hard disk" (does anyone know what this means?). I click ok then the I get the shutdown dialog with the "What do you want to do with your virtual hard disks?" and an option of "Commit changes to the virtual hard disk" etc.

I have checked the vmc files in notepad and all the paths seem to be ok. I've also used the virtual disk editor to check the parent disk references, all looks to be in order. The VM I'm trying to start uses a base disk which uses another base disk which uses another base disk. All of the base disk are present in the expected location.

Thanks for reading, any help gratefully received.

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2 Answers 2

Differencing disks are a way to "branch" multiple VMs starting at the same place. You set up a disk exactly how you want it, and then start moving forward with a "difference" disk, which literally keeps the differences between the base disk and your changes.

The problem is, you have to "freeze" the parent disk. If you make any changes to the parent, then the difference disk is toast. Most people will at least mark a parent difference disk as read only (or, even better, use security to prevent anybody from writing to the disk).

Once you've messed this up, there's no going back. All children are unusable. Unless you have a backup that can restore the parent disk to its original state, you're out of luck.

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Brad is completely correct. Once you've created differencing disks off of a parent VHD, you should never, ever touch the parent again. Any changes to it will invalidate the parenting chain for the diff disks, as you've seen.

There is no way to recover from this without a backup, or without recreating all of your diff disks.

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