Tutorial how to convert Windows System disk from MBR partitioning scheme to GPT (GUID Partition Table) without data loss in Windows Server 2012 R2.

This tutorial is targeted for VMWare virtual machine Windows Server 2012 R2, but it should be compatible with other Windows x64 editions (Vista/2008 and newer).

Update: (Checkout new article where Microsoft released official tool for MBR2GPT conversion)

Something about GPT before we start conversion:

• Allows up to 128 primary partitions
• Support for large partitions greater than 2 TB (MBR limit)
• Better workflow and CRC partition table protection
• Can be used as a boot volume on x64 Windows Server 2008/Vista and newer with UEFI
• Supports all x64 platforms

This tutorial is just for my personal purposes. It was published for people which knows what they do and needs fast solution in this way. I’m not responsible for any harm or damage of your OS and HDD.

• What we will need:
  • Installation Disc/USB/ISO of current installed Windows
  • UEFI support (WMWare has option to edit VM and switch from BIOS to EFI)
  • I assume you have supported system for booting from GPT, in this case I have Windows Server 2012 R2 with snapshot or system state backup if something goes wrong
  • Small tool named gptgen – (local mirror – gptgen-1.1)
• Find number of system disk (usually marked as Disk 0)

cmd
diskpart
list disk

• Download & run gptgen.exe from command line and accept warning with system disk conversion

gptgen.exe -w \\.\\physicaldrive0

•  Restart machine and boot to mounted Windows installation disc
• Navigate to Repair > Troubleshoot > Command prompt
• Identify system disk (usually marked as Disk 0 – you can also recognize it with “*” inside GPT column)

diskpart
list disk
select disk 0
list partition

• Select and delete system partition (it shoud have size of 350 MB with 1024 KB offset, in some situations it can be smaller, but not smaller than 230 MB!, then you will must resize system partition with some external tool for minimal supported partition size – 230MB)

select partition 1
delete partition

•  Create new Microsoft reserved partition (boot partition)

create partition EFI size=100 offset=1
format quick fs=fat32 label="System"
assign letter=S
create partition msr size=128 offset=103424

•  Now we need to assign volume letter where is installed our windows OS (it should has assigned letter in default, you can recognize it with no Label defined in default state)

list volume
select volume 2
assign letter=C

•  Exit DISKPART and generate new bootable partition data to our volume where is installed windows OS (check again if you’ve set correct letter for system volume, in this case letter C)

bcdboot c:\windows /s s: /f UEFI

•  Shutdown machine and enable booting with (U)EFI instead of BIOS

Done.

P.S. 1: You can also use gptgen for conversion from MBR to GPT to non-system disk without data loss. Simply run gptgen.exe -w \\.\\physicaldriveX where X is your desired non-system disk, then reboot.

P.S. 2: Method from “P.S 1” was also used to convert non-system 2TB MBR SAN Disk holding SQL Instance + SQL data. Conversion was successful. Just MSSQL service went down during conversion, so it needed to be manually started up as a service again.