Hard disk drives become much bigger and it's common to use a 2TB or 4TB single disk. Many servers are built with hardware RAID arrays, the virtual disk may be larger than 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 initialized as MBR, you can only use 2TB space, the remaining 4TB disk space is shown as "unallocated" at the end. This space cannot be used to create new volume or expand other partition. To use full disk space, you must convert disk from MBR to GPT. If there are 4 partitions on a disk and Windows doesn't allow creating more volume, you also need to change MBR disk to GPT. This article introduces how to convert MBR to GPT in Windows Server 2012 R2 with MBR2GPT command tool and disk conversion software.

To convert MBR disk to GPT in Windows Server 2012 R2, there are several ways. Windows has built-in Disk Management and diskpart command tool to help convert disk type, but you must on this MBR disk in advance.

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

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

If there's only data on the MBR disk, it is extremely easy and fast to convert it to GPT without losing data, only several clicks are needed.

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

If there's operating system on the MBR disk, it's a bit complicated, because all system files must keep intact and the OS must boot correctly after converting to GPT. There are some third party software that claim to be able to convert MBR disk to GPT with operating system in it, but few can ensure 100% system bootable. If you want to convert system disk from MBR to GPT in Server 2012 R2, it is suggested to run MBR2GPT.exe which is a command prompt tool provided by Microsoft.

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 in Server 2012 R2, you need bootable media with Windows Preinstallation Environment (Windows PE).

1. There should be no logical drive on this MBR disk.
2. There should be at most 3 primary partitions on this MBR disk.
3. One of the partitions is set as "Active" and is the system partition.
4. All partitions on this MBR disk are supported by Windows, that means there are no EXT2/EXT3 and other types of Linux or Mac partitions.

. This is the most common error message while converting MBR to GPT in Windows Server 2012 and other versions. If you received such error, check your disk partition configuration again.

As I said above, MBR2GPT.exe is built in Windows Server 2019 and Windows 10, it is not included in eariler Windows versions. To run MBR2GPT in Server 2012 R2, you need Windows Preinstallation Environment and run command prompt from it.

Download Windows Server 2019 ISO and create bootable DVD or USB flash drive with Windows built-in or third-party tool. If you use VMware, Hyper-V virtual machine, you just need to select this ISO file and boot from it.

Boot from this bootable media, simply click in first Windows Setup window, then click on th bottom left corner of next window.

Click in next window, then click .

Just input 2 commands to complete converting.

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

Because I extended the System Reserved partition in advance, restart server and boot to UEFI, disk 0 is converted to GPT and the EFI partition is created on left side of C drive.