Microsoft

How to convert existing partitions for use with UEFI firmware

Converting existing partition tables no longer requires repartitioning your drive, resulting in data loss. Windows 10 allows admins to convert partition types from WinPE or the OS-level itself.

Many systems - even newer machines - still run the Master Boot Record (MBR) partition scheme. Though thoroughly compatible with legacy BIOS systems, MBR is antiquated and its limitations are slowly catching up with it. By contrast, the GUID Partition Table (GPT) was developed as a layout for storage devices that are part of the Unified Extensible Firmware Interface (UEFI) - the modern and more secure successor to BIOS available on newer systems.

While overcoming the limitations of its predecessor, GPT is not utilized as often as MBR due to the requirement of using UEFI over legacy BIOS. However, as technology continues to advance, storage devices larger than 2TB or multi-boot systems running Windows, macOS, and Linux are requiring GPT and UEFI or else the system partitions will simply not boot.

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In the past, this meant formatting your drives with the GPT scheme and reinstalling the OSes so they conform to the UEFI firmware requirements. And while seasoned IT pros will have system data backed up before making any kind of changes to a drive's structure, the potential loss of data and time in migrating Windows systems is over, thanks to the MBR2GPT.exe utility found in Windows 10.

This small utility can convert an existing MBR partition currently in use, to the GPT scheme on the fly, either by booting the device to external media using WinPE or directly from the OS itself when logged on with administrative credentials. Before we jump right in, there are a few things you'll need:

  • PC running Windows 10 (build 1703) or later
  • Administrative credentials
  • WinPE external media (Optional)

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1. After logging on to the computer, launch CMD with UAC elevation and authenticate as an administrator.

2. Enter the following command to perform a validation on the system disk. The process checks to see if the conversion is possible without completing the process:

mbr2gpt.exe /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. If the process is validated successfully, then the system is eligible to be converted. To proceed with the process, execute the command below to perform the conversion from MBR to GPT:

mbr2gpt.exe /convert /allowfullos

MBR2GPT will now attempt to convert disk 0.
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!

Note: The /allowfullos switch is only required when executing the command from a live Windows environment. If executing from WinPE, you may omit this switch as it is deprecated.

4. Once the command has completed successfully, the computer will need to be rebooted. Also, verify the firmware settings and modify it to enable UEFI mode or else the Windows installation will not boot until the change has been made.

Also see...

Does your organization utilize UEFI firmware? If so, what are your reasons for doing so? Share your thoughts below in the comments section.

About Jesus Vigo

Jesus Vigo is a Network Administrator by day and owner of Mac|Jesus, LLC, specializing in Mac and Windows integration and providing solutions to small- and medium-size businesses. He brings 19 years of experience and multiple certifications from seve...

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