Use Microsoft’s built-in MBR2GPT.exe to convert an eligible Windows system disk from MBR to GPT without deleting its existing partitions. First validate the disk, then convert it from Windows PE or from full Windows with /allowFullOS. Afterward, switch the computer’s firmware to UEFI boot mode; conversion alone does not change that setting.
Before you convert: check compatibility and protect your data
MBR2GPT is included with supported Windows 10 and Windows 11 installations in WindowsSystem32. It converts a Windows system disk; it is not the tool for converting an arbitrary non-system data disk. The computer must support UEFI, and its firmware must be set to UEFI after conversion. Microsoft advises confirming UEFI support before starting: Microsoft’s MBR2GPT documentation.
- Back up important files and make sure you have a recovery plan before changing the system disk. Microsoft says MBR2GPT does not modify or delete data, but that is not a guarantee against every failure.
- If the disk has BitLocker-protected volumes, suspend BitLocker protection before conversion. After conversion, delete and recreate the existing protectors before resuming protection.
- Identify the Windows system disk and its disk number. If you omit
/disk, MBR2GPT defaults to the system disk; specifying the verified number reduces the chance of targeting the wrong disk.
What validation checks
Run /validate before /convert. Conversion will not proceed if the disk fails the eligibility checks. Microsoft’s documented checks include:
- The disk is MBR-formatted and has no more than three primary partitions.
- There are no extended or logical partitions, and one partition is active and designated as the system partition.
- There is enough unpartitioned space for GPT metadata: 16 KB plus 2 sectors at the beginning of the disk, and 16 KB plus 1 sector at the end.
- The system partition’s BCD store has a default operating-system entry pointing to an OS partition.
- Windows can retrieve volume IDs for each volume with an assigned drive letter.
- Partition types are recognized by Windows, unless a needed type mapping is supplied with
/map.
Choose where to run MBR2GPT
The commands depend on whether you run the tool in Windows PE or in the full Windows installation. The full Windows option requires /allowFullOS.
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| Environment | Validation command | Conversion command | EFI system partition behavior |
|---|---|---|---|
| Windows PE | mbr2gpt /validate /disk:0 |
mbr2gpt /convert /disk:0 |
The tool can reuse a suitable existing system partition or create an EFI partition as needed. |
| Full Windows | mbr2gpt /validate /disk:0 /allowFullOS |
mbr2gpt /convert /disk:0 /allowFullOS |
The in-use MBR system partition cannot be reused, so the tool creates a new EFI system partition by shrinking the OS partition. |
In these examples, replace 0 with the disk number you verified. Run commands in an elevated Command Prompt. Omit /allowFullOS in Windows PE.
Run validation, then convert
- Open an elevated Command Prompt. In Windows, search for Command Prompt, choose Run as administrator, and approve the prompt. If using Windows PE, open its command prompt.
- Validate the target disk. In Windows PE, run
mbr2gpt /validate /disk:0. In full Windows, runmbr2gpt /validate /disk:0 /allowFullOS. Substitute your verified disk number. - Proceed only if validation succeeds. If it reports that the disk layout is not valid or another check fails, do not run conversion. Resolve the reported issue or inspect the logs and return code.
- Convert the same disk. In Windows PE, run
mbr2gpt /convert /disk:0. In full Windows, runmbr2gpt /convert /disk:0 /allowFullOS. Keep the disk number the same as the one you validated. - Read the completion output before restarting. MBR2GPT validates, creates an EFI system partition if needed, installs UEFI boot files, applies GPT metadata and layout, updates BCD, and restores drive-letter assignments.
The documented command form is MBR2GPT /validate|convert [/disk:<diskNumber>] [/logs:<logDirectory>] [/map:<source>=<destination>] [/allowFullOS]. Use /logs:<directory> to choose a log location; the directory must already exist. Otherwise, logs default to %windir%. Use /map only when a partition type is not recognized by Windows and you know the appropriate mapping.
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Switch firmware to UEFI and verify Windows boots
After conversion, restart into the device’s firmware setup and change the boot mode from Legacy/CSM to UEFI. The exact menu name and navigation vary by manufacturer; consult the device maker’s instructions if you cannot identify the setting. Do not assume that MBR2GPT changes firmware configuration for you.
Once Windows starts, check that it reports UEFI boot mode. Microsoft’s deployment test guidance also recommends checking Secure Boot state and confirming that applications continue to function: Microsoft’s MBR2GPT test instructions. Secure Boot status is a verification item; the conversion procedure does not itself establish that Secure Boot is enabled.
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When an existing system partition can be reused
Outside the full-Windows /allowFullOS case, MBR2GPT may reuse the existing system partition as the EFI system partition only if it is not also the OS or Windows Recovery Environment partition, is at least 100 MB (260 MB on 4K-sector disks), and is no larger than 1 GB. If those conditions are not met, the tool may need to create an EFI partition. With /allowFullOS, the existing MBR system partition is in use and cannot be reused.
Troubleshoot a failed validation or conversion
If validation fails, conversion does not proceed. Start with the exact error, return code, and log rather than retrying with different options blindly. Microsoft’s reference lists return-code categories for cancellation, initialization or internal failures, invalid parameters, disk geometry or layout problems, encryption, EFI partition creation, boot-file installation, GPT layout application, and cases where conversion succeeds but some BCD entries cannot be restored.
- Confirm that the disk number is the Windows system disk you intended to target.
- Check the documented partition, BCD, free-space, and volume-ID requirements above.
- If BitLocker is involved, verify protection was suspended for the conversion.
- If conversion reports an EFI partition or boot-file problem, use the log and exact return code to identify the failed stage before attempting recovery.
- If conversion completes but Windows does not boot, verify firmware is set to UEFI and follow the device manufacturer’s firmware guidance.
MBR2GPT versus DiskPart “convert gpt”
| Tool | Intended use | Existing partitions and data |
|---|---|---|
| MBR2GPT | Convert an eligible Windows MBR system disk for UEFI boot. | Designed to preserve the existing data layout; validation must pass first. |
DiskPart convert gpt |
Convert an empty basic MBR disk to GPT. | The disk must be empty; partitions or volumes must be deleted first. |
DiskPart’s command is therefore not a substitute when you want to convert the existing Windows system disk while keeping its current layout. See Microsoft’s DiskPart convert gpt reference.
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