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A few years back, the Windows operating system was commonly installed on a Master Boot Record (MBR) partition. However, with the security advancements in firmware (UEFI) and much larger disks, GUID partition table (GPT) disks are now required to take advantage of these new features. If you have a system disk configured as an MBR disk, the MB42GPT.exe utility can convert your disk to GPT.
GPT vs. MBR disks
The newer GPT disks provide advantages over MBR disks and allow the use of more secure and performant UEFI firmware. Note the following advantages:
- UEFI compatibility
- More than four partitions on each disk are supported
- Support for disks larger than 2 terabytes (TB)
Needless to say, the GPT partition style is the best partition type moving forward, with modern hardware and operating systems taking advantage of UEFI firmware and large disks.
What is MBR2GPT.exe?
MBR2GPT.exe is a tool that converts disks from the older MBR partition table to the newer GUID partition table. It does not require deleting any of the data on the disk to make the change. MBR2GPT.exe is included in Windows 10 and Windows 11 installations.
There are two ways to run the command:
- Windows Preinstallation Environment (Windows PE) command prompt
- Run from Windows 10 or Windows 11 using the /allowFullOS option
If you type mbr2gpt.exe, you will see the output from the command displayed. Note the following options:
- Validate—Validates that the disk can be converted
- Convert—Validates that the disk can be converted and performs the conversion
- Logs—Specifies the directory for logging and defaults to %windir%
- Map—Specifies the GPT partition type to be used for the MBR partition if it isn't recognized by Windows
- allowFullOS—Allows the tool to run from the full Windows environment rather than the default Windows PE requirement
Here, we are running the command on a Windows 10 22H2 installation to view the parameters:
MBR2GPT capabilities and prerequisites
With MBR2GPT, you can:
- Transform any connected MBR-formatted system disk into the GPT partition style. However, this tool does not support the conversion of nonsystem disks from MBR to GPT.
- Convert an MBR disk that contains BitLocker-encrypted volumes, provided that protection is currently suspended. After conversion, you must remove the current protectors and establish new ones to continue using BitLocker.
- Alter operating system disks installed with earlier versions of Windows 10, such as versions 1507, 1511, and 1607.
- Transform an operating system disk from MBR to GPT through the Configuration Manager or MDT, as long as your task sequence employs Windows PE version 1703 or later.
- The disk is presently configured with MBR
- Sufficient unoccupied space exists to house both the primary and secondary GPTs, specifically:
- 16 KB plus 2 sectors at the disk's beginning
- 16 KB plus 1 sector at the disk's end
- The MBR partition table contains no more than three primary partitions
- One partition is designated as active and functions as the system partition
- There is an absence of any extended or logical partition on the disk
- The BCD store located in the system partition includes a default OS entry, which directs data to an OS partition
- The volume IDs can be obtained for each volume possessing an assigned drive letter
- Every partition on the disk corresponds to MBR types that Windows recognizes or has a specified mapping through the use of the /map command-line option
Windows 10 MBR disk conversion
Suppose you have a Windows 10 installation with an MBR partition table. You might want to convert your disk to GPT to use UEFI firmware. UEFI has many benefits, such as much better security, faster startup and shutdown times, and the ability to turn on modern security features, such as Secure Boot and Credential Guard.
You can verify you are running an MBR partition for your system disk by launching disk management (type diskmgmt.msc at a run or search box), right-click, and choose Properties.
Next, click the Volumes tab of the disk properties dialog box that pops up. You will see the partition style listed. Here, we see Master Boot Record (MBR).
The first thing we can do is validate the partition using the following command:
mbr2gpt.exe /validate /allowFullOS
This process will check to see whether the disk can be converted and ensure that the layout of the disk and disk sectors can be read.
After we have validated the partition for conversion, we can run the conversion from the MBR disk to GPT using the following command:
mbr2gpt.exe /convert /allowFullOS
The conversion completes successfully.
At this point, you need to switch your computer from booting using BIOS firmware to using UEFI. Restart Windows, and then boot into your startup settings. In most computers, you will boot into your SETUP screen and modify your BOOT options to change the setting from BIOS to UEFI.
For virtual machines, this is similar. It is generally a property of your advanced startup options.
In VMware Workstation, this setting is found under your Virtual Machine settings > Advanced > Firmware Type:
Changing a virtual machine from BIOS to UEFI
Once you change your firmware type to UEFI, Windows should again be able to boot after converting from MBR to GPT.
Now, we can verify that the partition table has been changed correctly to GPT. Open Disk Management again (diskmgmt.msc from a run or search menu), and select the properties of your disk.
Below, the disk correctly shows that it is using the GPT partition table (GPT).
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The MBR2GPT tool makes it easy to switch from a Master Boot Record (MBR) to a GUID partition table (GPT) partition style. There are a few prerequisites to remember, such as that the disk must be a system disk and contain an active partition. Nonetheless, it makes converting from the older MBR partition style to the newer GPT style possible so that you can take advantage of the latest security and performance features with UEFI firmware and large disks over 2 TB.