If you didn't know Microsoft now includes MBR2GPT.exe allowing you to convert a disk from the Master Boot Record (MBR) to the GUID Partition Table (GPT) partition style without modifying or deleting data on the drive. It's easy and quick to do although there are some ifs, ands, and buts. Here's all you need to know including what we've experienced and what Microsoft advises.
According to Microsoft, MBR2GPT can:
Convert any attached MBR-formatted system disk to the GPT partition format. You cannot use the tool to convert non-system disks from MBR to GPT.
Convert an MBR disk with BitLocker-encrypted volumes as long as protection has been suspended. To resume BitLocker after conversion, you will need to delete the existing protectors and recreate them.
Convert operating system disks that have earlier versions of Windows 10 installed, such as versions 1507, 1511, and 1607. However, you must run the tool while booted into Windows 10 version 1703 or later, and perform an offline conversion.
Convert an operating system disk from MBR to GPT using Configuration Manager or MDT provided that your task sequence uses Windows PE version 1703 or later.
While MBR2GPT.EXE will work with most MBR partitions, skim this list to be sure you can convert:
MBR2GPT.EXE requires Windows 10 version 1703 or newer. Previous operating systems, including Windows 7 and 8.1 are not supported.
If you're using BitLocker encryption, be sure to suspend it before you begin. If you're not sure if you are, it's a good bet that you're not.
The disk is currently using MBR
There is enough space not occupied by partitions to store the primary and secondary GPTs: - 16KB + 2 sectors at the front of the disk - 16KB + 1 sector at the end of the disk
There are at most 3 primary partitions in the MBR partition table. 4+ partitions will return an error.
One of the partitions is set as active and is the system partition
The disk does not have any extended/logical partition
The BCD store on the system partition contains a default OS entry pointing to an OS partition
The volume IDs can be retrieved for each volume which has a drive letter assigned
All partitions on the disk are of MBR types recognized by Windows or has a mapping specified using the /map command-line option
If any of these conditions are not met, then you will get an error.
While MBR2GPT.EXE is easy to use, we recommend only advanced users try this or get help. You could also create a system image before you begin or at least back up your important data. Knowledge of the Command Prompt and PowerShell, switches and how to identify your partition is a necessity.
Type in mbr2gpt to see a list of available switches or commands. For your reference, they are:
/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.
Take note of the allowFullOS option. If you're running from a command-prompt, this is required. If running from Windows PE, this switch isn't needed.
A simple test is to validate your drive. From the command prompt type in mbr2gpt /validate /disk:0.
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