Disk2VHD: The 127 GB File size limit. UNMOUNTABLE_BOOT_VOLUME

In a previous post I talked about the Disk2VHD file conversion tool. Here is an “Oh Golly Gee Darn” moment.

(This blog is G rated however the rediscovery rates stronger wording.)


There were really two problems with the disk I created. Both problems were fixable.

  1. The VHD file was too large for use with Virtual PC 2007 which has an IDE drive size limit of 127 GB.
  2. In creating the file I copied all of the partitions from the previous XP box. However in installing Windows 7, I only reformatted the OS drive and installed Windows 7 v64. Therefore, the XP VHD contained all of the Data partitions as well as the OS partition. The actual hardware had the orginal data partitions as well as the new OS partition. When I attempted to mount the VHD in Windows 7 using the native support for VHD the drive signatures were the same and created a signature collision. However I could mount the VHD on different hardware without a problem.


The tool give no warning about the 127 GB file size limit Vrtual PC has for files. This is a big problem if you intend to boot from the created file. Big problem if you just used the tool to create your Virtual PC XPmode before going to Windows 7 64bit. So what can you do?

What to do if you created a VHD file larger than 127 GB as a backup of your PC before reformating.

First, I empathize with the situation and it is an easy trap to fall in to. You say to yourself, it worked for smaller sized drives so there was no reason to doubt it would work with larger drives. The example on the Disk2VHD page even uses a larger native drive but has less used space than 127 GB. So one day the example file will stop working if it grows beyond 127 GB. That is a pretty lame problem.

So what can you do? Hyper-V can use larger VHD files. And so can Windows 7. In part the solution is that you can mount your VHD file in Windows 7 and then rerun the tool to copy out individual drives to individual files. My post on Native support for VHD files will come in handy for you on this.

What to do if your VHD has “signature collision” with the “host computer” when trying to mount drives using native support for VHDs in Windows 7 or Windows 2008 R2.

It is important to understand what is happenning in order ot understand how to solve the problem. In my case I had created a perfect copy of the drives on the physical computer using Disk2vhd. Then I deleted only one of the disk partitions (The OS Disk). When I reinstalled the OS in the partition I reformatted I now had a new OS with the old partitions. I also had the VHD which included the old OS partition as well as an exact copy of the physical partiitions I had on the host OS.

So when I went to mount the partitions in the VHD file, the OS recognized the the disk signatures as being identical in the VHD as on the host system.

The fix: Mount the VHD on another Windows 7 box as read only and run Disk2VHD again breaking out the partitions as individual files.

If you need more help feel free to contact me.

Jeff Loucks
Available Technology
Available Technology
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