Convert MBR to GPT with the new MBR2GPT conversion tool

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Windows 10 build 1703 introduces a new console tool called MBR2GPT, which lets you convert an MBR disk (Master Boot Record) to a GPT disk (GUID Partition Table) without data loss or modification. MBR is an old method of partitioning disks that uses a special boot sector at the beginning of the partitioned storage to identify the location of the bootable operating system.

PCs with BIOS used to have MBR prior to the newer UEFI standard, which gave rise to GPT. It refers to the standard layout for partition tables using globally unique identifiers (GUIDs). Prior to the release of the Creators Update, users had to choose between MBR or GPT at the time of formatting the disk. That means changing the partition table style would erase data on the disk.

With MBR2GPT in the Creators Update, users can now convert an existing MBR disk to a GPT disk without deleting it. MBR2GPT.exe runs from a Windows Preinstallation Environment command prompt as well as from a regular Windows 10 copy. You can start the console tool with a special set of arguments.

The syntax of the command is:

MBR2GPT /validate|convert [/disk:] [/logs:] [/map:=] [/allowFullOS]

The command line parameters are described as follows:

  • /validate: Instructs MBR2GPT.exe to perform only the disk validation steps and report whether the disk is eligible for conversion.
  • /convert: Instructs MBR2GPT.exe to perform the disk validation and to proceed with the conversion if all validation tests pass.
  • /disk: Specifies the disk number of the disk to be converted to GPT. If not specified, the system disk is used. The mechanism used is the same as that used by the diskpart.exe tool SELECT DISK SYSTEM command.
  • /logs: Specifies the directory where MBR2GPT.exe logs should be written. If not specified, %windir% is used. If specified, the directory must already exist, it will not be automatically created or overwritten.
  • /map: Specifies additional partition type mappings between MBR and GPT. The MBR partition number is specified in decimal notation, not hexidecimal. The GPT GUID can contain 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 overrides this block and enables disk conversion while running in the full Windows environment.

The tool currently supports conversion of disks for Windows 10 versions 1507, 1511, 1607 and 1703.

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