Convert MBR disk to GPT without data loss

This tutorial explains how convert an MBR disk to GPT without data loss using the new tool MBR2GPT Windows 10.

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GPT is a new generation format that allows a disk (HDD or SSD) to have up to 128 partitions and a maximum size of 256 To by partition.

MBR2GPT can only convert system disks (disks on which Windows is installed). If you want to convert data disks to GPT and without data loss, use Gptgen.

Important : if your goal is to convert the system disk (the one where Windows is installed) from your computer, make sure that it has a firmware UEFI. Indeed, only UEFI can start on a GPT disk. If you convert your system disk to GPT while you have BIOS firmware, you will no longer be able to boot into Windows.

MBR2GPT: presentation of the tool

MBR2GPT is a tool that allows you to convert an MBR (Master Boot Record) type disk to GPT (GUID Partition Table) without modifying or deleting data on the disk.

The tool is designed to be run from a command prompt from the Windows pre-installation environment (Windows PE) (i.e. from the Windows 10 installer accessible via a W10 installation USB key) but it can also be run from Windows 10 using the option /allowFullOS.

MBR2GPT is available on Windows 10 v1703 and later versions of W10.


You can use MBR2GPT for:

  • Convert a disk system MBR to GPT. You you can not use the tool to convert non-system disks (Windows must be installed on the disk to convert).
  • Convert an MBR disk with volumes encrypted by BitLocker if protection is suspended. To use BitLocker again after conversion, you will need to remove the existing protectors and recreate them.
  • Convert the system disks on which older versions of Windows 10 are installed (1507, 1511 and 1607). However, you need to run MBR2GPT from Windows 10 v1703 or later and convert offline (i.e. from WinPE).

Converting system disks that have older versions of Windows installed (Windows 7, 8 or 8.1) is not not officially supported. The recommended way to convert these disks is to upgrade Windows 7 or 8.1 to Windows 10 and then convert MBR disk to GPT, or else use Gptgen.

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MBR2GPT syntax

MBR2GPT /validate|convert [/disk:] [/logs:] [/map:=] [/allowFullOS]
Option Description
/validate Tells MBR2GPT.exe not to perform that the validation steps disk and indicate whether the disk is eligible for conversion.
/convert Instructs MBR2GPT.exe to perform disk validation and convert if all validation tests pass.
/disk: Specifies the disc number to convert to GPT. If not specified, the system disk is used. The mechanism used is the same as that used by the SELECT DISK SYSTEM command of the diskpart.exe tool.
/logs: Specifies the directory in which newspapers MBR2GPT.exe must be written. If not specified,% windir% is used. If specified, the directory must already exist, it will not be created or replaced automatically.
/map:= Specifies additional mappings. The MBR partition number is shown in decimal, not hexadecimal. The GPT GUID can contain square brackets, for example: / map: 42 = {af9b60a0-1431-4f62-bc68-3311714a69ad}. Multiple / map options can be specified if multiple mappings are required.
/allowFullOS By default, MBR2GPT.exe is blocked unless it is run from Windows PE. This option disables this blocking and allows you to convert a disk in a Windows environment running.
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Convert an MBR disk to GPT

From Windows PE

Here is how to use MBR2GPT to convert a system disk to GPT offline, that is, from the Windows Pre-Installation Environment (WinPE):

  1. Open a Command Prompt since :
  2. Identify the number system disk to convert using DiskPart:
    diskpart
    list disk
  3. Check whether the disk can be converted to GPT (replace 0 by the number of your disc):
    mbr2gpt /validate /disk:0
    
    MBR2GPT: Attempting to validate disk 0
    MBR2GPT: Retrieving layout of disk
    MBR2GPT: Validating layout, disk sector size is: 512 bytes
    MBR2GPT: Validation completed successfully
  4. Convert the system disk in GPT:
    mbr2gpt /convert /disk:0
    
    MBR2GPT will now attempt to convert the default book disk.
    If conversion is successful the disk can only be booted in GPT mode.
    These changes cannot be undone!
    
    MBR2GPT: Attempting to convert disk 0
    MBR2GPT: Retrieving layout of disk
    MBR2GPT: Validating layout, disk sector size is: 512 bytes
    MBR2GPT: Trying to shrink the system partition
    MBR2GPT: Trying to shrink the OS partition
    MBR2GPT: Creating the EFI system partition
    MBR2GPT: Installing the new boot files
    MBR2GPT: Performing the layout conversion
    MBR2GPT: Migrating default boot entry
    MBR2GPT: Adding recovery boot entry
    MBR2GPT: Fixing drive letter mapping
    MBR2GPT: Conversion completed successfully
    MBR2GPT: Before the new system can boot properly you need to switch the firmware to boot to UEFI mode!
  5. Once the disk has been converted to GPT, restart your PC then go to the UEFI firmware settings.
  6. In order for your computer to boot in UEFI mode, change the “UEFI Boot” option to UEFA instead of Legacy (or CSM).
  7. Save the changes then restart your computer to boot to Windows 10 in UEFI mode.

Congratulations! You have converted an MBR system disk to GPT using the MBR2GPT tool and since WinPE. 👍

From Windows 10

Here is how to use MBR2GPT to convert a system disk to GPT from a Windows 10 running :

  1. Open a Command Prompt as administrator.
  2. Check whether the disk can be converted to GPT (replace 0 by the number of your disc):
    mbr2gpt /validate /allowFullOS
    
    MBR2GPT: Attempting to validate disk 0
    MBR2GPT: Retrieving layout of disk
    MBR2GPT: Validating layout, disk sector size is: 512 bytes
    MBR2GPT: Validation completed successfully
  3. Convert the system disk in GPT:
    mbr2gpt /convert /allowFullOS
    
    MBR2GPT will now attempt to convert the default book disk.
    If conversion is successful the disk can only be booted in GPT mode.
    These changes cannot be undone!
    
    MBR2GPT: Attempting to convert disk 0
    MBR2GPT: Retrieving layout of disk
    MBR2GPT: Validating layout, disk sector size is: 512 bytes
    MBR2GPT: Trying to shrink the system partition
    MBR2GPT: Trying to shrink the OS partition
    MBR2GPT: Creating the EFI system partition
    MBR2GPT: Installing the new boot files
    MBR2GPT: Performing the layout conversion
    MBR2GPT: Migrating default boot entry
    MBR2GPT: Adding recovery boot entry
    MBR2GPT: Fixing drive letter mapping
    MBR2GPT: Conversion completed successfully
    MBR2GPT: Before the new system can boot properly you need to switch the firmware to boot to UEFI mode!
  4. Once the disk has been converted to GPT, restart your PC then go to the UEFI firmware settings.
  5. In order for your computer to boot in UEFI mode, change the “UEFI Boot” option to UEFA instead of Legacy (or CSM).
  6. Save the changes then restart your computer to boot to Windows 10 in UEFI mode.

Congratulations! You have converted an MBR system disk to GPT using the MBR2GPT tool and from a Windows 10 running. 👍

Other tools

To convert a data disk (non-system) to GPT, you can use either DiskPart (with loss of data) or Gptgen (without data loss). For more information :

Convert MBR disk to GPT on Windows 10, 8 and 7

Useful links

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