Convert MBR to GPT on Windows Server 2012 R2

Updated on: November 16, 2019

This article introduces how to convert MBR to GPT on Windows Server 2012 (R2) without losing data. 2 Ways to change MBR disk to GPT with mbr2gpt command prompt and NIUBI partition editor.

Convert MBR to GPT option greyed out

Hard disk drives become much bigger, to the servers that use hardware RAID arrays such as RAID 5, the virtual disk may be over 10TB. If you build a RAID 5 array with 3 3TB hard disks, the virtual disk in Disk Management is 6GB. If this disk is MBR style, you can only use 2TB space, the extra 4TB Unallocated space cannot be used to create new volume or expand other one. In addition, you can't create a single volume larger than 2TB in a MBR disk.

To convert MBR to GPT on Windows Server 2012 (R2), there are several ways. Windows has built-in Disk Management and diskpart command tool to help convert. However, you must delete all partitions in the MBR disk before converting.

In diskpart command prompt window, type help convert GPT and you'll see the syntax, note and example of convert command.

Diskpart Convert

In Disk Management console, when you right click the front of a MBR disk, there is also an option. However, the "Convert to GPT Disk" option is grayed out if there's a partition in it.

Convert is grayed

With MBR2GPT.exe (Microsoft provided) and NIUBI Partition Editor, you can convert MBR to GPT without losing data. Before starting, make sure the MBR disk that you want to convert is data disk or system disk, because the method is different.

How to convert MBR to GPT for non-system disk

To data only disk, it is extremely easy to convert MBR to GPT in Windows Server 2012 (R2), only several clicks are needed.

  1. Download NIUBI Partition Editor, right click the front of the MBR disk and select "Convert to GPT Disk".
  2. Simply click Yes to confirm, then a pending operation is added.
  3. Click Apply on top left to execute, done. (All operations before clicking Apply only work in virtual mode.)

Watch the video how to convert MBR disk to GPT in Windows Server 2012:

Video guide

There are some 3rd-part software that can convert MBR to GPT on Windows Server 2012, but few can ensure 100% system bootable. If you want to convert a MBR system disk to GPT, it is suggested to run MBR2GPT which is provided by Microsoft.

Server 2012 mbr2gpt.exe command prompt

MBR2GPT.exe is running from command prompt. On Windows Server 2019 and Windows 10 (1703 and later versions), you can convert MBR to GPT with this command in Windows. But to run mbr2gpt on Server 2012, you need bootable media with Windows Preinstallation Environment (Windows PE).

To convert with MBR2GPT command, your disk partition should meet for the requirements, otherwise, the conversion can't be successful. So before downloading this tool, check the disk prerequisites:

  1. The disk is currently using MBR
  2. There is enough space not occupied by partitions to store the primary and secondary GPTs: 16KB + 2 sectors at the front of the disk and 16KB + 1 sector at the end of the disk.
  3. There are at most 3 primary partitions in the MBR partition table
  4. One of the partitions is set as active and is the system partition
  5. The disk does not have any extended/logical partition
  6. The BCD store on the system partition contains a default OS entry pointing to an OS partition
  7. The volume IDs can be retrieved for each volume which has a drive letter assigned
  8. All partitions on this MBR disk are recognized by Windows.

In general, you just need to pay attention to number 3, 5 and 8.

Before any change to the disk is made, MBR2GPT validates the layout and geometry of the selected disk, if any of these checks fails, the conversion will not proceed and an error message will be returned.

Common issue and corresponding solution

"Validating layout, disk sector size is: 512 bytes Disk layout validating failed for disk 0", "MBR2GPT: Conversion failed". This is the most common error message while converting MBR disk to GPT. If you received such error, check your disk partition configuration again.

Partition layout

In most of Windows 2012 server system disk, there are System Reserved, C: (for OS) and D drive. If all these partitions are Primary, you can convert this disk successfully.

Note: If there are more than 3 partitions, or even one Logical partition, or any non-support partition such as EXT2/3 in the disk, you can't convert MBR disk to GPT in Windows Server 2012.

Pay attention to EFI partition

For Windows to remain bootable after the conversion, an EFI system partition (ESP) must be in place. MBR2GPT will shrink the System Reserved partition first. If there is not enough free space in it, MBR2GPT will shrank the OS C drive. In that case, the EFI partition will be created on the right side of C drive, which causes serious issue, for example: C drive cannot be extended forever by either Windows native or 3rd-party software.

As you see, 100MB EFI system partition is created behind C drive after converting Disk 0 to GPT. When right clicking it, all options are grayed out.

EFI partition

Download NIUBI Partition Editor and follow the method in the video to expand the System Reserved partition (it's enough to 1GB):

Video guide

Convert MBR to GPT on Server 2012 for system disk

After expanding the System Reserved partition, there is plenty of free space, MBR2GPT will shrink this partition to create EFI partition on the left side of C drive. Now everything is ready.

Steps to convert mbr to gpt on Windows Server 2012 (R2):

Step 1: Download Windows Server 2019 ISO and create bootable DVD disc or USB flash drive with Windows built-in or third-party tool.

Step 2: Boot from this bootable media, simply click Next in first Windows Setup window, then click Repair your computer on bottom left corner of next window.

Setup window

Repair computer

Step 3: Click Troubleshoot in next window, then click Command Prompt.


Command Prompt

Step 4: Just input 2 commands to complete converting.

  1. cd..
  2. mbr2gpt /convert

As you see, the command window reports conversion completed successfully.

  • /validate - Instructs MBR2GPT.exe to perform only the disk validation steps and report whether the disk is eligible for conversion. If validating failed, you cannot continue converting.
  • /convert - Instructs MBR2GPT.exe to perform the disk validation and to proceed with the conversion if all validation tests pass.
  • /disk:<diskNumber> - Specifies the disk number of MBR disk to be converted to GPT. If not specified, the system disk is used, so this option is not used here.

See full Syntax and options

Restart server and boot to UEFI, as you see, disk 0 is converted to GPT and the EFI system partition is created on left side of C drive.

Besides converting MBR to GPT for Server 2012, NIUBI Partition Editor helps you convert partition between Primary and Logical, convert NTFS to FAT32 without losing data. It also helps you shrink, extend, move, merge, copy, wipe partition and much more.