Convert MBR to GPT on Windows Server 2012 (R2)

This article introduces how to convert MBR to GPT on Windows Server 2012 (R2) without data loss. Change MBR disk to GPT with mbr2gpt command prompt and partition editor

Convert MBR to GPT 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.

To convert MBR to GPT on Windows Server 2012 (R2), there is an option in the native Disk Management utility. However, when I right click the front of a MBR disk, the option “Convert to GPT Disk” is grayed out.

Convert is grayed After deleting all partitions of this disk and retry, “Convert to GPT Disk” option becomes available.

Convert available

Obviously, this method is not good. If you have a blank new MBR disk, you can do like this. We need a way to convert MBR to GPT without data loss.

Server 2012 mbr2gpt command prompt

There are some 3rd part software that can convert MBR to GPT on Windows Server 2012, but few can ensure system bootable. Luckily, Microsoft provides an official tool – MBR2GPT to accomplish this task.

MBR2GPT.exe is running from command prompt. On 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). However, to convert a MBR disk without Operating System, it is much easier and I’ll show you later.


  • Before converting MBR to GPT disk, you should confirm that your server support booting to UEFI (difference between UEFI and BIOS)
  • After converting system disk to GPT, it can’t be undone, so make sure you must convert.
  • Back up server before converting.
  • Not all system disk can be converted to GPT, the disk partition configuration must fit for the requirements.

Disk Prerequisites

Before any change to the disk is made, MBR2GPT validates the layout and geometry of the selected disk to ensure that:

  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
    • 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 the disk are of MBR types recognized by Windows or has a mapping specified using the /map command-line option

If any of these checks fails, the conversion will not proceed and an error will be returned.

In this list, you should pay attention to number 3, 5 and 8.

Check in Server 2012 Disk Management:

Right click Windows on bottom left corner of the screen and select Disk Management, and then you’ll see how many partitions in a disk, file system and type of each partition.

Tips It doesn’t matter if you use single physical disk or RAID array virtual disk.

There are 3 Primary partitions in Disk 0, so converting this MBR disk to GPT is supported.

But if there are 4 Primary partitions or even one Logical partition in Disk 0, you can’t convert this MBR disk to GPT on Server 2012.


If there are other types of partitions such as EXT2/3 in this MBR disk, it cannot be converted. Check the supported partitions.

Convert MBR to GPT on Server 2012 for system disk

It is very easy to do this, but there is one point you should concern.

Important: 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.

Tips Neither Disk Management nor 3rd party software can shrink, extend, move or edit this EFI system partition. As you see, right click it in Disk Management, all options are grayed out.

Download NIUBI Partition Editor and follow the step in the video to expand the System Reserved partition.

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

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

Step 1 Download Windows 10 setup tool from https://www.microsoft.com/en-us/software-download/windows10 and select the second option to create installation media with it.

Burn to USB disc, DVD or build ISO file and burn later with your own tools. (Note: Use blank media or transfer files before burning.)

Step 2 Restart server and boot from this USB or DVD, when it ask you to “Install now”, do NOT click it, click “Repair your computer” on bottom left instead.

Click Troubleshoot in the next window.

Click Command Prompt in the next window.

Step 3 You just need 2 commands to convert system disk to GPT: (see full Syntax and options)

mbr2gpt /convert
  • /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.

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.

How to convert MBR to GPT for non-system disk

It is extremely easy to fast to convert mbr to gpt on Windows Server 2012 (R2) for non-system disk. You just to run NIUBI Partition Editor, select the MBR disk and convert. Watch the video:

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, etc.