Posts Tagged ‘MBR’

  1. Create a system repair disc (  ). You can skip this step if you have a Windows installation media. Is a good measure to reboot and verify you can start your system from this disc.
  2. Identify which disk you want to convert (usually is #0). This can be done by looking at the number in the Windows Disk Management.
  3. Download gptgen from here  . (You could use the Windows Disk Management Tool for this too. (diskmgmt.msc)) This tool will allow you to convert your MBR disc to GPT with the data included. 
  4. ATTENTION: After this step, your computer CANNOT BOOT until the whole process is completed. DO NOT Shut Down Unless Instructed to! 

    Unzip gptgen and then run CMD with elevated privileges. (replace the 0 with the identified disk number).

    This *will* result in a BSOD shortly after and it’s to be expected:

    gptgen.exe -w \\.\physicaldrive0

  5. Boot using your Windows installation or previously generated system repair disc.
  6. Choose language and preferences, and then select Repair Your Computer -> Troubleshoot -> Advanced options -> Command Prompt
  7. We will need the disk partitioning tool. With this, we will recreate the boot partitions. Type:


  8. Identify the boot disk where Windows is located, typing:

    list disk 

     Something like this should appear:


          Disk ###  Status         Size     Free     Dyn  Gpt

          --------  -------------  -------  -------  ---  ---

        * Disk 0    Online          128 GB      0 B        *

  9. Once identified, select the disk (replace with the correct number):

    select disk 0

  10. Verify the partitions:

    list partition

  11. Something similar at the info below should appear.

           Partition ###  Type              Size     Offset

           -------------  ----------------  -------  -------

           Partition 1    Primary            350 MB  1024 KB

           Partition 2    Primary            126 GB   350 MB

  12. Delete the previous system partition:

      select partition 1

      delete partition

  13. Create the new boot partition, Microsoft reserved partition:

      create partition EFI size=100 offset=1

      format quick fs=fat32 label="System"

      assign letter=S

      create partition msr size=128 offset=103424

  14. If you list the partitions again, you should have ended up with something like this:

           Partition ###  Type              Size     Offset

           -------------  ----------------  -------  -------

           Partition 1    System             100 MB  1024 KB

           Partition 2    Reserved           128 MB   101 MB

           Partition 3    Primary            126 GB   229 MB

  15. Ensure that your Windows installation is mounted, replacing 3 with the volume number of the Windows installation (usually 1):

      list volume

      select volume 3

      assign letter=C

  16. Exit diskpart:


  17. Generate boot partition data, replacing C: with the letter of the Windows installation (usually C:):

      bcdboot c:\windows /s s: /f UEFI

  18. Cross your fingers and then restart your computer!


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Had to do this for server over the weekend for a data disk which had reached the 2TB size limit imposed on MBR disks and couldn’t really find a lot of information on how to achieve this without data loss.

Starting from Windows 10 there is a built-in tool called MBR2GPT.exe which does exactly what it says – converts MBR disks to GPT, but for versions of Windows prior to this (i.e. Windows 7/8/8.1 or 2008 R2/2012/2012 R2) Microsoft’s official stance is to backup the data, delete the partitions  on the disk, convert the disk to GPT then restore the data. Not exactly practical.

While there are a number of third party tools that can claim to do this, a lot of them aren’t free. I was able to find a simple and reliable tool called gptgen.exe that could convert the disk to GPT format with the server online and without causing any data loss. The tool can be found here:

To use the tool:

  • Identify the disk you need to modify. You can find the disk ID by running diskpart > list disk or just look in Disk Management
  • To test gptgen without making any changes to the disk, run the following command in an elevated command prompt window:
    gptgen.exe [file://.//physicaldriveX]\\.\\physicaldriveX – where X = the disk ID for the disk you want to convert
  • If the test command above runs without any issues, you can run the below command to write the changes to the partition table:

gptgen.exe -w [file://.//physicaldriveX]\\.\\physicaldriveX

NB: make sure you have a valid backup/snapshot prior to running the above command and that you stop all relevant services on the device.

  • Give the server a reboot then confirm any services or programs housed on the converted disk start correctly
  • Check the “Volumes” tab in the properties of the converted disk in Disk Management and you should see it’s now a GPT disk

Also worth noting that Microsoft officially supports GPT data disks running on machines in legacy BIOS mode, but in an ideal world you’d want to convert the machine to the newer UEFI BIOS mode for the extra features, improved performance, etc. etc. This will  involve converting the OS disk from MBR to GPT and is a bit more of an involved process. Best to speak to your nearest Senior Consultant about this as the steps can vary on a case by case basis.


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When you install Windows using a Legacy Device is marks the Boot Volume with an Master Boot Record (MBR) label

I use easybcd to load all my ISO’s on a single USB which doesn’t support UEFI 

You need GUID Partition Table (GPT) for UEFI Boot

You can check this via Disk Management

Restart the PC into Safemode with Command Prompt ( Or Hold down Shift when you click on Restart on the login screen )   

*You will need Local Admin password for this!

Run this command to make sure the drive can support GPT 

mbr2gpt /validate

Then Run this 

mbr2gpt /convert

After this you need to change the Boot Loader to use UEFI instead of Legacy


To do this with a System with Bitlocker

Suspend Bitlocker before hand then reboot in command prompt

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