SSD Stuck in MBR? Convert to GPT in One Command

Why GPT — and Why You Back Up First

MBR is the old partition layout: it caps a disk at 2 TB, allows only four primary partitions, and boots through Legacy BIOS mode. GPT is the modern layout — it pairs with UEFI firmware, and UEFI is what Windows 11 expects. If Windows was installed in Legacy mode, even a fast NVMe SSD stays stuck on MBR, and the usual "fix" people suggest is a full reinstall. It is not needed: Microsoft ships a tool called MBR2GPT with Windows 10 and 11 that converts a system disk from MBR to GPT in place, keeping your Windows, apps and files exactly where they are.

Confirm the Disk Is MBR and Note Its Number

First prove the disk really is MBR — do not convert on a guess. Press Win + R, type the command below and press Enter to open Disk Management:

diskmgmt.msc

In the lower pane, right-click the grey box of your Windows disk (it will say Disk 0, Disk 1, and so on) → Properties → Volumes tab. The Partition style line tells you the truth: "Master Boot Record (MBR)" means this guide applies; "GUID Partition Table (GPT)" means the disk is already converted and you can stop here. Note the disk number carefully — the conversion commands need it, and on most PCs the Windows disk is Disk 0.

The same check works from DiskPart. Open Command Prompt and run:

diskpart
list disk
exit

In the list disk output, look at the Gpt column: an asterisk (*) means the disk is already GPT. No asterisk on your Windows disk confirms it is MBR.

Check the Current Boot Mode

Press Win + R, type the command below and press Enter to open System Information:

msinfo32

Find the BIOS Mode line. On an MBR system it reads Legacy — that is the "before" state. After the conversion and the firmware change in Step 6, this same line should read UEFI, which is how you will know the whole job is done. If it already says UEFI while the disk is MBR, note that down too — it means the firmware is ready and only the disk layout is behind.

Validate the Disk Before Converting

Never run the conversion blind. Open Command Prompt as administrator (right-click Start → Terminal (Admin) or Command Prompt (Admin)) and first run the validation pass. Replace 0 with your disk number from Step 2 if it is different:

mbr2gpt /validate /disk:0 /allowFullOS

The /allowFullOS switch is what lets the tool run from inside your working Windows; without it, MBR2GPT only runs from a WinPE/recovery environment. Validation changes nothing on the disk — it only checks whether the layout can be converted safely. When it reports that validation completed successfully, the disk is a good candidate and you can go ahead.

Run the One Command — Convert to GPT

With validation passed, run the conversion in the same admin Command Prompt:

mbr2gpt /convert /disk:0 /allowFullOS

That single command does the whole job: it shrinks the Windows partition slightly, creates the EFI System Partition, converts the partition table from MBR to GPT, and rewrites the boot configuration so Windows will start from the new GPT layout. Your data, apps and settings are not touched. Let it finish completely — do not close the window, restart, or let the machine sleep while it runs.

Switch the Firmware to UEFI and Reboot

This is the step people forget — and the one that decides whether the PC boots. A GPT system disk can only boot in UEFI mode, so before (or immediately after) the first restart, enter your BIOS/UEFI setup (usually Del, F2 or F12 at power-on, depending on the brand), find the boot mode setting, and switch it from Legacy / CSM to UEFI. Save and exit, and Windows should start normally — now from the GPT disk.

Verify the Conversion Worked

Once Windows is up, repeat the two checks from Steps 2 and 3. In Disk Management (diskmgmt.msc), open the disk's Properties → Volumes tab: Partition style should now read GUID Partition Table (GPT). In System Information (msinfo32), BIOS Mode should now read UEFI. Both flipped means the job is complete — your SSD is on the modern layout, ready for UEFI features and Windows 11 requirements, with everything you had still in place.