To convert a Windows system disk from MBR to GPT without deleting its data, use Microsoft’s built-in MBR2GPT.exe: back up important files, check eligibility with /validate, convert only if validation succeeds, then switch the PC’s firmware to UEFI boot mode. The tool is designed not to modify or delete disk data, but a backup is still prudent; validation is not a guarantee against every machine-specific problem.
Which conversion method preserves data?
For an eligible Windows system disk, Microsoft’s MBR2GPT tool converts the partition style without modifying or deleting data. It is built into supported Windows installations at WindowsSystem32 and can run from Windows PE or from full Windows with the /allowFullOS option.
| Route | Data preservation | Disk scope | Required follow-up |
|---|---|---|---|
MBR2GPT.exe |
Designed to preserve disk data; validate first and keep a backup. | Eligible Windows system disks only. | Switch firmware to UEFI; handle BitLocker protection if enabled. |
| Windows Setup wipe and reformat | Deletes partitions and erases their data. | Disk used for a clean Windows installation. | Boot Setup media in UEFI mode and install Windows afresh. |
DiskPart convert gpt |
Not a data-preserving shortcut: Microsoft’s command requires an empty basic disk. | Empty basic disks. | Partitions and volumes must be deleted first. |
Microsoft documents the Setup wipe route in its Windows Setup partition-style guide and the empty-disk requirement for DiskPart’s convert gpt command. Neither is equivalent to MBR2GPT for converting a populated system disk without erasing it.
Check whether the system disk is eligible
MBR2GPT converts a Windows system disk, not an arbitrary storage or data disk. Before running it, identify the disk that contains Windows; the /disk:<number> option selects the target, while leaving the option out defaults to the system disk.
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Microsoft’s validation checks that the selected disk meets requirements including:
- It uses MBR and has no more than three primary partitions.
- It has an active system partition and no extended or logical partitions.
- There is enough unallocated space for GPT metadata: 16 KB plus two sectors at the front of the disk and 16 KB plus one sector at the end.
- The BCD store has a valid default OS entry.
- Volume IDs can be retrieved for volumes with drive letters, and partition types are recognized or explicitly mapped.
If the disk fails any required check, validation stops the conversion. Do not proceed by deleting partitions or guessing at a partition-type mapping to force eligibility.
Prepare, validate, and convert
1. Prepare the PC
- Back up important data before changing the disk layout.
- Confirm that the computer supports UEFI firmware boot. After conversion, Windows must boot in UEFI mode.
- If BitLocker is enabled, suspend protection before conversion and make sure you can access recovery information through your normal device-management process. Encryption does not have to be disabled.
2. Validate the selected disk
Open an elevated Command Prompt. In Windows PE, run:
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mbr2gpt /validate /disk:<diskNumber>
From the full Windows environment, include /allowFullOS:
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Replace <diskNumber> with the number of the Windows system disk. Validation checks eligibility without converting the disk; a successful result is reported as “Validation completed successfully.”
3. Convert only after validation succeeds
In Windows PE, run:
mbr2gpt /convert /disk:<diskNumber>
In full Windows, run:
mbr2gpt /convert /disk:<diskNumber> /allowFullOS
The conversion command validates again and proceeds only if its checks pass. If validation fails, stop and investigate the reported reason rather than trying conversion anyway.
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4. Set firmware to UEFI
Once conversion completes, enter the computer’s firmware setup and change the boot mode to UEFI. The setting’s name and location vary by manufacturer, so use the device maker’s instructions rather than assuming a universal keypress or menu path. Microsoft says the firmware must be reconfigured to boot in UEFI mode after the disk is converted.
5. Confirm Windows starts and restore BitLocker protection
Boot Windows in UEFI mode and confirm it starts, the disk is GPT, and important data and applications are accessible. If BitLocker protection was suspended, delete and recreate the existing protectors before resuming protection, as Microsoft specifies.
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The tool validates the layout, creates or reuses an EFI System Partition, installs UEFI boot files, applies GPT metadata and layout, updates BCD, and restores drive-letter assignments. When run from full Windows, the existing system partition is in use and cannot be repurposed as the EFI partition.
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If an existing system partition qualifies for reuse, it must not also be the OS or Windows Recovery Environment partition. It must be at least 100 MB (260 MB on a 4K-sector disk) and no larger than 1 GB. Otherwise, MBR2GPT shrinks the OS partition to create a FAT32 EFI System Partition sized 100 MB (260 MB on a 4K-sector disk).
Microsoft’s disk-scheme overview describes GPT as supporting disks larger than 2 TB; capacity limits depend on the specific Windows and disk context, so that figure should not be treated as a universal maximum. See Microsoft’s disk partition-scheme overview for the comparison.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.If validation fails or conversion reports an error
Stop if validation fails. Read the reported reason and inspect the partition layout and boot configuration for the requirement that was not met. Common documented areas to check are partition count, extended or logical partitions, the active system partition, the BCD default OS entry, available space, and unrecognized partition types.
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MBR2GPT can write diagerr.xml, diagwrn.xml, setupact.log, and setuperr.log; Microsoft identifies setupact.log and setuperr.log as the most detailed process and layout logs. To specify a log location, use /logs:<directory>; that directory must already exist.
If the tool cannot restore a drive letter, it reports an error or warning for manual intervention. Microsoft notes this can happen after the layout has been converted and cannot be undone at that stage. Preserve and inspect the logs rather than assuming the operation will automatically roll back.
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