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On a supported Windows 10 or Windows 11 system disk, Microsoft’s built-in MBR2GPT.exe can convert the partition table from MBR to GPT without deleting the files on the disk. The conversion is only half the job: after it succeeds, restart, open the firmware settings, and change boot mode from Legacy BIOS or CSM to UEFI. If you run the procedure on a non-system data disk, however, MBR2GPT is not supported; that disk must be backed up, erased, repartitioned as GPT, and restored.
Back up first, verify the disk number and layout, suspend BitLocker if enabled, validate before converting, and keep the conversion logs if anything fails. The steps below cover the normal offline method in Windows PE or WinRE, the /allowFullOS alternative, firmware changes, validation errors, and the separate destructive workflow for data-only disks.
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What does converting MBR to GPT change, and why might you need it?
MBR and GPT are partition-table formats. MBR uses 32-bit Logical Block Addressing and has a maximum addressable disk size of 2 TiB (2,199,023,255,552 bytes), according to Microsoft Azure Support in 2026. GPT uses 64-bit LBA and supports disks up to 18 exabytes. Those are partition-table addressing limits; the usable capacity of a particular disk is also constrained by its firmware, operating system, controller, and drive support.
GPT is also the partition style expected by UEFI-based booting. Converting a Windows installation to GPT lets you move from Legacy BIOS/CSM boot to UEFI without reinstalling Windows, provided the machine’s firmware supports UEFI and the system-disk layout passes MBR2GPT’s checks.
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Can MBR2GPT convert a Windows disk without deleting files?
Yes, for an attached Windows system disk that meets the eligibility rules. Microsoft describes MBR2GPT as converting the disk “without modifying or deleting data on the disk.” It changes the partitioning and boot arrangement in place; it is not a substitute for a backup.
MBR2GPT cannot convert a non-system data disk. A disk containing only photographs, project files, or other data requires a different process that erases the disk after a backup. Do not select a data disk in the system-disk procedure simply because it is large or has a familiar drive letter.
What should I check before running MBR2GPT?
Complete these checks before opening the conversion tool. They prevent the most common failures and give you a recovery path if the computer has an unrelated hardware or boot problem.
Make a recoverable backup
- Create a current backup or recovery image of the Windows installation and important files. Microsoft’s Azure procedure takes an OS-disk snapshot before conversion; a local recovery image serves the same purpose on a physical PC.
- Confirm that you can access the backup or recovery environment before changing the partition table. A backup that has never been tested is not a reliable recovery plan.
Confirm UEFI support
Open the computer’s firmware documentation or setup utility and verify that it offers UEFI boot. MBR2GPT’s conversion can complete on a machine that still has Legacy mode selected, but Windows will not boot from the converted disk until the firmware is switched to UEFI.
Identify the correct disk and partition style
In an elevated PowerShell window, run:
Get-Disk | ft -AutoRecord the number of the Windows system disk and confirm that its partition style is MBR. The disk number used in every command must be the system disk’s number, not merely the number of a data disk you intend to convert.
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Handle BitLocker
If BitLocker protects the Windows volume, suspend protection before validation and conversion. Microsoft allows conversion while protection is suspended. After conversion and a successful UEFI boot, resume protection by deleting and recreating the existing BitLocker protectors as Microsoft specifies; simply assuming that the old protectors will remain valid can leave the volume in an unexpected protection state.
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MBR2GPT requires a layout that can host UEFI boot files and GPT metadata. The system disk must have no more than three primary partitions, no extended or logical partition, an active system partition, a valid default operating-system entry in the Boot Configuration Data (BCD), and recognized partition types. It also needs free space for GPT metadata: 16 KB plus two sectors at the front of the disk and 16 KB plus one sector at the end.
How do I validate and convert the system disk offline?
The normal and most conservative route is to run MBR2GPT from Windows PE or the Windows Recovery Environment, where the Windows system partition is not in ordinary use. Use an elevated Command Prompt and replace 0 below with the disk number you confirmed.
- Boot to Windows PE or WinRE. Use your organization’s recovery media or the computer’s Windows recovery options, then open Command Prompt with administrator rights.
- Validate without changing the disk. Run:
mbr2gpt /validate /disk:0Validation checks the partition count, partition types, BCD, active system partition, available space, and other conversion prerequisites. It does not modify the disk.
- Read the result. Continue only when MBR2GPT reports that validation completed successfully. If it fails, stop and use the troubleshooting section rather than trying the conversion command repeatedly.
- Convert the disk. Run:
mbr2gpt /convert /disk:0The conversion command validates again, creates or reuses an EFI System Partition, installs UEFI boot files, writes GPT metadata, updates BCD entries, and restores drive-letter assignments.
- Restart immediately after success. Do not continue working in the recovery command window as though the machine were still configured for Legacy boot. Restart and enter the firmware setup.
The documented syntax also accepts /logs:<directory> to choose a log location and /map:<source>=<destination> to map an MBR partition type to a GPT type when a specialized layout requires it. Use those switches only when you understand the mapping required by the disk; they do not bypass the core eligibility checks.
Can I run MBR2GPT while full Windows is running?
Yes, but use this option only when offline execution is impractical. In an elevated Command Prompt, add /allowFullOS to both commands:
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mbr2gpt /validate /disk:0 /allowFullOSmbr2gpt /convert /disk:0 /allowFullOSIn the full operating system, the existing MBR system partition is in use. MBR2GPT therefore creates a new EFI System Partition by shrinking the Windows OS partition. That shrink operation is the principal difference from the offline path, so ensure the OS partition has enough movable free space and avoid interrupting the process.
What exactly happens during conversion?
Microsoft describes six phases:
- The tool validates the disk and Windows boot configuration.
- It creates an EFI System Partition when an appropriate existing system partition cannot be reused.
- It installs UEFI-compatible boot files.
- It applies GPT metadata and the new partition layout.
- It updates the BCD store for UEFI booting.
- It restores drive-letter assignments.
If the existing system partition is suitable, MBR2GPT may reuse it. Otherwise, it creates a 100 MB EFI System Partition by shrinking the OS partition; on a disk with 4K sectors, the EFI partition is 260 MB.
How do I switch the computer from Legacy BIOS to UEFI after conversion?
After MBR2GPT reports success, restart and enter the firmware setup using the key shown by the manufacturer, commonly a function key, Delete, or Escape. Find the boot-mode setting and change it from Legacy BIOS or CSM to UEFI. Save the change and reboot.
Select the Windows Boot Manager entry associated with the converted disk if the firmware presents a boot-order list. GPT conversion can be complete even when the firmware still points at a legacy boot entry, so a missing or incorrect Windows Boot Manager selection is a common reason for a post-conversion boot failure.
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What does “MBR2GPT validation failed” mean?
It means the disk did not meet one or more prerequisites; it does not identify a single universal problem. The usual causes are too many primary partitions, an extended or logical partition, an unsuitable or inactive system partition, an invalid default BCD operating-system entry, an unrecognized partition type, insufficient room for GPT metadata, or encryption that was not suspended.
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Use the logs before changing the layout
MBR2GPT writes diagerr.xml, diagwrn.xml, setupact.log, and setuperr.log. Microsoft identifies setupact.log and setuperr.log as the most detailed files for disk-layout and conversion errors. Review those files from the log directory before deleting a partition, shrinking a volume, or rerunning a command.
Match the symptom to a safe response
- Partition-count or layout error: inspect the disk in Disk Management or the recovery environment and determine which partition violates the requirements. Do not remove a recovery or vendor partition until you have a verified backup and know its purpose.
- BCD or boot-entry error: repair the Windows boot configuration first. A valid default OS entry is required; conversion cannot reliably create UEFI boot files from a broken BCD store.
- Encryption error: suspend BitLocker and verify that protection is actually suspended before trying validation again.
- Insufficient-space error: free or consolidate space in the location identified by the log. The required GPT metadata space is small, but the full-OS path may also need room to shrink the OS partition for the EFI System Partition.
- Wrong disk number: stop immediately, rediscover the disk with
Get-Disk | ft -Auto, and rerun validation against the intended Windows disk only.
What do MBR2GPT return codes tell me?
A return code of 0 means conversion completed successfully. Code 100 means the conversion succeeded but some boot-configuration entries could not be restored; treat that as a boot-repair task and inspect the logs rather than assuming every entry survived.
Other return codes indicate cancellation, invalid parameters, an ineligible layout, encryption, EFI-partition creation failure, boot-file installation failure, or failure while applying GPT metadata. The exact code should be read together with setupact.log and setuperr.log; a code alone rarely tells you which partition or boot entry needs correction.
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Return to the firmware setup first and confirm that the machine is in UEFI mode and that the Windows Boot Manager entry on the converted disk is selected. If those settings are correct, boot to Windows recovery media, preserve the MBR2GPT logs, and inspect the boot configuration and conversion messages before attempting another repair.
Do not switch randomly between Legacy and UEFI modes while troubleshooting. The converted disk is intended to boot through the UEFI files and BCD entries that MBR2GPT installed; changing modes without checking the firmware entry can obscure the original fault.
How is a data-only MBR disk converted to GPT?
There is no nondestructive MBR2GPT path for a non-system disk. Microsoft’s data-disk procedure is a backup-and-recreate workflow and erases the target disk.
- Copy the data to a separate disk or other verified backup location and confirm that the copy can be opened.
- Open an elevated Command Prompt and start DiskPart.
- Select the intended disk by its number, using the disk listing to avoid selecting the Windows disk.
- Run DiskPart’s
cleancommand. This removes the existing partition information and makes the data inaccessible until it is restored. - Run
convert gpt, create a new partition, and format it with the file system and label appropriate for your use. - Restore the backed-up files and verify permissions, drive letters, and application paths.
Because clean is destructive, a data-disk conversion should be scheduled as a migration, not treated as an in-place upgrade. If you cannot temporarily store the data elsewhere, do not proceed.
Which conversion method should I choose?
| Situation | Recommended method | Data impact | Important conditions |
|---|---|---|---|
| Attached Windows system disk | Offline mbr2gpt /validate, then /convert |
In-place; designed not to delete disk data | UEFI firmware, eligible partition layout, backup, and BitLocker suspension when applicable |
| Windows system disk when WinPE/WinRE is unavailable | Use both commands with /allowFullOS |
In-place; the OS partition may be shrunk to create the EFI partition | Run elevated, ensure movable free space, and avoid interruption |
| Non-system data disk | Back up, DiskPart clean, convert gpt, recreate, format, and restore |
Destructive; existing partitions and data are erased | Requires a verified backup and enough temporary storage |
What are the safest final checks?
- Windows starts normally in UEFI mode after the restart.
- The firmware boot order names Windows Boot Manager on the converted disk.
- Disk management tools report the system disk as GPT.
- BitLocker protection is resumed only after the new UEFI boot path works and the protectors have been recreated as required.
- The MBR2GPT logs and recovery image are retained until you have completed at least one normal reboot and confirmed access to your files and applications.
Frequently Asked Questions
Can MBR2GPT convert an external USB drive or a second internal drive?
MBR2GPT is intended for the attached Windows system disk, not a general-purpose data-drive converter. A second internal disk or an external USB disk should be handled with the backup, erase, GPT-recreate, and restore workflow instead.
How much space does MBR2GPT need for GPT metadata?
The documented minimum is 16 KB plus two sectors at the front of the disk and 16 KB plus one sector at the end. The EFI System Partition is separate: MBR2GPT creates 100 MB on ordinary-sector disks or 260 MB on 4K-sector disks when it cannot reuse a suitable system partition.
Can I return to Legacy BIOS after converting to GPT?
The converted installation is configured for UEFI boot, so changing firmware back to Legacy normally prevents that boot path from working. Returning to an MBR/Legacy arrangement requires a separate backup-and-repartition migration rather than an MBR2GPT undo command.
Should I delete the old system partition after conversion?
No. Do not remove partitions simply because their names or drive letters look unfamiliar. MBR2GPT may reuse an existing system partition, and the conversion logs should be reviewed before any partition is changed.
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