Your Windows disk is MBR, you want UEFI and GPT, and reinstalling everything is not an afternoon you have. Microsoft quietly ships the tool for exactly this: MBR2GPT converts the disk in place. Used in the right order, your Windows, apps and files stay exactly where they were.
MBR2GPT is a command-line tool built into Windows 10 (version 1703 and later) and Windows 11. It reworks an MBR system disk into GPT: it shrinks the Windows partition slightly, creates the EFI system partition, rewrites the partition table and installs the UEFI boot files — without formatting. It also has strict rules and one trap at the end that leaves PCs unbootable for people who stop reading early. This page is the full safe order: check the rules, validate, convert, then flip the firmware.
👆 Example output pattern. Validation must complete successfully first — never convert a disk that failed it.
Not sure the disk number? Run diskpart then list disk first. On most single-disk PCs the system disk is disk 0, but "most" is not a standard to convert by.
MBR2GPT is designed to be non-destructive, and in practice it is reliable — designed and guaranteed are different words. Take a backup or at least copy irreplaceable files elsewhere before any partition-table surgery. Then open Command Prompt as administrator inside the running Windows and validate:
mbr2gpt /validate /disk:0 /allowFullOSmbr2gpt /convert /disk:0 /allowFullOS/allowFullOS switch is what permits running it from inside the full Windows you are using. From a WinPE or recovery command prompt, omit that switchYour disk is now GPT, but your PC is still booting in legacy mode — and a legacy firmware cannot boot a GPT Windows. Restart, enter setup, and change the boot mode to UEFI (turn CSM/Legacy off). While you are there you can also enable Secure Boot, which this conversion now makes possible. Only then let it boot. Skip this step and the PC reports no bootable device, which is the single most common "MBR2GPT broke my PC" story — the conversion was fine; the switch was missed.
The video explains MBR and GPT from the ground up — after converting, watch it once and you will understand exactly what state your disk and firmware are now in.
▶️ Watch Full VideoUsually the disk has the maximum partitions already, or an unusual layout from an old upgrade. Free a slot (a vendor recovery partition is the common candidate, after imaging it), or accept that this disk needs the backup-convert-restore path instead. Do not force the conversion past a failed validation.
No — it targets the system disk by design. For a data disk, the supported route is backup, convert (which wipes), restore. There is no in-place Microsoft converter for non-system disks, and third-party tools that promise it should be judged by whether you have that backup.
There is no reverse MBR2GPT. GPT-to-MBR in place is not offered by Windows tools. Going back means wiping and converting, or restoring your pre-conversion backup — one more reason the backup step on this page is not decoration.