How can I migrate a Windows 10 from BIOS/MBR boot to UEFI/GPT without reinstalling?

Question

I've installed Windows 10 in BIOS mode and use MBR. How can I convert it to use UEFI and GPT without reinstalling Windows 10? Is this possible?

Answer 1

In Windows 10 v1703 (Creators Update, Build 15063), there is a tool called MBR2GPT.EXE included which does all steps.

Best is to boot from a Win10 v1703 DVD/USB drive to the repair option (WinPE) and run Command prompt (cmd.exe) under repair options. Now use diskpart and list command to see all drives.

X:\>diskpart

Microsoft DiskPart version 10.0.15063.0

Copyright (C) Microsoft Corporation.
On computer: Win10v1703

DISKPART> list disk

  Disk ###  Status         Size     Free     Dyn  Gpt
  --------  -------------  -------  -------  ---  ---
  Disk 0    Online           60 GB      0 B 

Now look or the disk number where your Windows is installed. This number is important for next step. Now run mbr2gpt /convert /disk:ID with the ID:

X:\>mbr2gpt /convert /disk:0

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!

Now, reboot the PC, enter the UEFI and change the mode from BIOS/legacy to UEFI only:

Now save the changes, reboot and Windows 10 should boot. When you run msinfo32.exe you should see that Windows 10 runs now in UEFI mode.

Note: You can also convert Windows 10 v1511, 1607 installations, but you need to boot from a 1703 ISO! Converting older Windows versions (7,8,8.1) is not supported by Microsoft. ALSO, always make a full backup of the PC, in case something goes wrong!

Answer 2

The tool to which magicandre1981 refers sounds interesting and useful, but I've never used it myself. If I had that latest Windows version and wanted to convert to EFI-mode booting, I'd probably try that tool.

If you've got an older version and don't want to upgrade, it is possible, but the procedure is complicated. In brief, you must convert from MBR to GPT using a tool that can do this losslessly and then install an EFI-mode boot loader for Windows. This blog post describes one procedure for doing this.

Whatever approach you use, I strongly recommend making a complete system backup, or at least backing up your personal files, before attempting such a conversion. Although the MBR-to-GPT conversion is surprisingly simple, there's always a risk of severe data loss when mucking about with partition tables and boot loaders, so having backups is important.