How do I Fix a Corrupted Virtual Hard Disk?

People occasionally contact me and ask me how to fix a virtual hard disk that they have which is reporting corruption. 

The short answer is: "Run data recovery tools inside the virtual machine."

For the long answer we need to look at how the virtual hard disk is structured.  Reading from the Virtual Hard Disk specification (available here) you can see that virtual hard disks have the following structure:

Breaking down the components of the virtual hard disk - there is:

• The hard drive footer:

  • The hard drive footer contains a bunch of information - like the size of the disk, the disk geometry, the disk creator, etc...  Fixed size virtual hard disks only have one copy of the hard drive footer - but dynamic and differencing disks make a second copy of the hard drive footer at the top of the virtual hard disk.

• The dynamic virtual hard disk header:

  • The dynamic virtual hard disk header contains information about where the block allocation table is, as well as some of the parameters of the block allocation table structure.  It also contains the information for differencing disks to point to appropriate data locations on the parent virtual hard disk.

• The block allocation table:

  • The block allocation table is used by dynamic and differencing virtual hard disk in order to map requests for virtual hard disk offsets to the actual location on the physical hard disk (this is not needed in fixed size virtual hard disks as they have a constant offset value).  Depending on the size of the virtual hard disk the block allocation table can range in size from a couple of KB to a couple of MB (8MB is the largest block allocation table I have seen).

• Data blocks:

  • This is the bulk of the virtual hard disk and is the raw data that the virtual machine has written to the virtual hard disk.

Now let us look at the implications of data corruption in each of these areas - and the possible actions you can take:

• The hard drive footer:

  • Stored in the hard drive footer is a CRC checksum.  If this footer is corrupted we will detect it immediately.  In the case of a dynamic or differencing virtual hard disk we will attempt to restore the footer using the mirror copy at the top of the file.  In the case of a fixed size virtual hard disk (or for a dynamic or differencing virtual hard disk where both the footer and mirror have been corrupted) you will be presented with an error saying that the virtual hard disk is corrupted when you try to start the virtual machine (or edit the virtual hard disk).

    There are no tools or methods available for fixing this sort of corruption - and your only option is to restore a copy of the virtual hard disk from your most recent backup (you are backing up - right?).  Keep in mind that the footer is only 511 bytes in size - so the chances of if getting corrupted are relatively low.

• The dynamic virtual hard disk header:

  • The dynamic virtual hard disk header uses a CRC checksum similar to the hard drive footer.  As there is no mirror - corruption of this section of the virtual hard disk will also result in an error message from Virtual PC / Virtual Server / Hyper-V that states that the virtual hard disk is corrupted.  There is no way to recover from this sort of corruption (apart from restoring a backup) - but like the hard drive footer, the chances of corruption here are low due to the fact that it is only 1KB in size.

• The block allocation table:

  • There is no checksum on the block allocation table.  As such, if this section of the virtual hard disk were to be corrupted - you would start seeing file system errors inside the virtual machine as it would be directed to the wrong section of the virtual hard disk to read data.  There is no way to correct this sort of corruption.

• Data blocks:

  • The data blocks are completely opaque to Virtual PC / Virtual Server / Hyper-V.  Which is to say that the guest operating system writes data to the disk, and we just pass it on.  We do not know what file system is being used or what sort of structures exist here.  As such Virtual PC / Virtual Server / Hyper-V will never detect or report corruption that happens inside the data blocks.  Rather this sort of corruption will be reported by the guest operating system (either through error messages and warnings in the guest operating system - or in extreme cases - through failure to boot).  The way to handle corruption here is to treat the virtual hard disk just like a corrupted physical hard disk - and to boot the virtual machine and run data / disk recovery tools inside the virtual machine.

With all of this information I can provide the "medium length answer" as to what to do with corrupted virtual hard disks, as follows:

• If Virtual PC / Virtual Server / Hyper-V report that the virtual hard disk is corrupt - then one of the header / footer sections has been corrupted - and there is nothing that you can do.
• If the virtual machine starts correctly - but then reports errors accessing the disk - run data recovery tools inside the virtual machine.
• Always take backups of important virtual machines.

One final point I would like to make here is this - if you ever suffer data corruption in a virtual hard disk it is important that you figure out how it happened.  The two most common causes I have encountered are:

• Data corruption due to faulty / failing physical storage
• Data corruption due to transferring files over faulty / failing networks

In either case you want to identify as fix the root cause as soon as possible - otherwise you are going to suffer further corruption and data loss in the future.  I strongly recommend starting by looking at the event log in the host / parent environments - as 90% of the time this will provide you with the information that you need.

Cheers,
Ben

• Anonymous
  September 23, 2011
  I have an dynamic VHD - after an reboot I lost 3 days of work. It look like the Headers have an error and not point to the correct  Data Block. Is there a way to correct the header?? Thanks a lot

• Anonymous
  November 12, 2012
  How do I run data recovery tools inside the virtual machine when I can't start up xp mode/

• Anonymous
  July 26, 2014
  just wanted to if a virtual disk that reports zero bytes means that header or footer is currupted

• Anonymous
  October 08, 2014
  If you compare the header of a good and a problem vhdx, with a hex editor, you can sometime see if it is 'minor' header corruption or something more massive Note the header is quite large, and largely full of 00's, so need to look quite far into it to find non zeros bits in the good version

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• Anonymous
  April 08, 2016
  Hi,Very intersting arcticle. I had 3 times a bad VHDX, VM oS was reporting errors on the disk and My host was timing out on the CSV my VM is on trying to access it.Because of that my cluster became extremely unstable, the VM unmanageable.I fixed it by isolating the VM on the the host.rebooting the host.Locate the VM as soon it fail-over another Host and shut it down.Clone the VM (new VHDX File)power up the new VMDelete the old one therefore deleting the bad VHDX.Hope it help anyone having the same problem i had.

  • Anonymous
    September 25, 2016
    This article helped me to recover my broken VHD, here is my story dedicated to anyone with similar problems:I accidently enlarged a VHD without knowledge of two snapshots taken two years ago. Thanks to 59manager who allowed this!!!As a result the .vhd file was enlarged whereas the .avhd files were not. I got error messages saying "broken chain" of the snaphost files.I deleted the snapshots, but nothing happens. Mounting the original VHD was possible, but the data was two years old.Unfortunately the last backup was three month ago.As a first step I recovered the backup into a new disk, but users weren't happy because of three month emails missing.So my goal was to fix the virtual disk. I tried to shrink the orginal VHD file without success, got errormessages like "UUID of the files do not match", "wrong sizeinformation"...After hours of googling I found a tool named 010 Editor. This is a binary editor and has a wonderful feature: Templates, which means it can interprete a file's content by programmable rules. and, luckily a template named VHD comes with it. After downloading the 30 days trial I was able to see (and edit) the content of the VHD header in a readable manner.Changing the value of the field "Current size" to the value of "Original size" gave the error message that the file is corrupted.So with the help of the Virtual Hard Disk specification, mentioned above, I tried to correct the checksum, which is the 1's complement of the total sum of all the footer bytes. I changed it the following way: - Calculate the 1's complement - Subtract the bytes of the old (larger) "Current size" field - Add the Bytes of the new "Current size" filed, which is equal to "the originalSize" - do the 1's complement againThis I had to do for both copies of the footer, oneat the beginning and one at the end of the file.And, after two nights of reasearch and data copying I had the luck of a functional VHD.Hope this was helpful .Michael