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When MBR2GPT fails on Windows 10, do not immediately rerun the same command. First confirm that you selected the Windows system disk, run /validate, read the MBR2GPT logs, and use the return code to identify whether the failure involves disk eligibility, encryption, EFI partition creation, boot-file installation, GPT metadata, or BCD restoration. Correct that condition, validate again, and convert only after validation succeeds.
Microsoft documents MBR2GPT as a conversion tool for a Windows system disk. It changes an MBR system disk to GPT without modifying or deleting data, but the machine must boot in UEFI mode afterward. A current backup and a suspended BitLocker state are prudent safeguards before changing the partition layout.
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What does “mbr2gpt failed” mean?
MBR2GPT reports failure when one phase of its documented workflow cannot complete. The message is not a diagnosis by itself: the useful information is the phase, return code, and log entry associated with the failure.
The tool performs several operations in sequence:
- It validates the selected disk and its partition layout.
- It creates a new EFI System Partition (ESP), or reuses a suitable existing partition.
- It installs UEFI boot files.
- It writes GPT metadata and updates the partition layout.
- It updates the Boot Configuration Data (BCD) and restores drive-letter assignments.
A failure in one stage can leave later stages untouched. Treat the result as a documented validation or conversion-phase problem, not as a reason to repeat the command blindly.
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How do I confirm that I selected the right disk?
MBR2GPT is for the Windows system disk; it is not a general-purpose converter for an arbitrary secondary data drive. Before running anything that changes the layout, identify the disk that contains the active Windows installation and verify that its partition style is MBR.
Use Disk Management
Open Disk Management from the Windows power-user menu and locate the disk containing the Windows volume (normally the volume assigned C:) and the system partition used to start Windows. Open the disk’s properties and check its partition style. Record the disk number exactly as Windows displays it; do not assume that the Windows installation is always on Disk 0.
Cross-check with DiskPart or PowerShell
DiskPart and PowerShell can provide a second view of disk numbers, partitions, and styles. Match the disk number to the Windows volume and system partition you identified in Disk Management. If more than one disk is installed, compare the capacity and partition contents in each utility before choosing the /disk: value.
Check firmware capability
The computer’s firmware must support UEFI. MBR2GPT changes the disk’s partition style, but it cannot make legacy-only firmware boot a GPT system disk. Confirm that the firmware setup offers a UEFI boot mode before conversion; you will need to switch from legacy BIOS or CSM to UEFI after a successful conversion.
How do I run MBR2GPT validation before conversion?
Validation is a check-only operation. It is the safest way to learn whether the selected disk meets the documented prerequisites.
From Windows PE or Windows Recovery Environment
At a Windows PE command prompt, run:
mbr2gpt /validate /disk:<diskNumber>Replace <diskNumber> with the number you confirmed in Disk Management, DiskPart, or PowerShell. Windows PE is the normal operating environment for MBR2GPT because the installed Windows system partition is not in active use there.
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From the full Windows 10 installation
If you must run from the ordinary Windows desktop, add /allowFullOS:
mbr2gpt /validate /disk:<diskNumber> /allowFullOSThe /allowFullOS switch permits execution in the full operating system, but it changes how MBR2GPT can create the EFI System Partition because the existing system partition is in use. A validation result obtained in the full OS should still be followed by conversion in the same supported context, with the environment’s limitations in mind.
Understand what a successful validation means
Validation does not convert the disk. It confirms that MBR2GPT can proceed with the disk’s current partition, boot, and metadata conditions. If validation reports an error, repair the reported condition and run validation again; do not use /convert as a test.
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Which disk and partition conditions cause validation to fail?
Microsoft’s validation rules are specific. A disk can look usable in File Explorer and still fail because of its partition map, boot configuration, or metadata space.
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- There must be enough unoccupied 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 disk can have no more than three primary partitions.
- An extended partition or logical partition cannot be present.
- A partition must be marked active and serve as the system partition.
- The system partition’s BCD must contain a valid default Windows operating-system entry.
- Volume IDs must be readable.
- Partition types must be recognized, unless an appropriate mapping is supplied with the supported mapping option.
These checks concern the whole system disk, not just the Windows volume. Recovery, tools, and vendor partitions count toward the primary-partition limit, and an extended/logical arrangement must be corrected before validation can pass.
EFI System Partition sizing
When MBR2GPT must create a new EFI System Partition, Microsoft specifies 100 MB on ordinary-sector disks and 260 MB on disks that use 4K sectors. The required free area and the way the existing system partition is used differ between Windows PE and a full Windows run with /allowFullOS, so an EFI-creation error should be diagnosed in the environment shown in the logs.
What to do when layout validation fails
- Recheck the disk number and confirm that it is the Windows system disk.
- Inspect the partition list for a fourth primary partition, an extended partition, or a logical drive.
- Confirm that the system partition is active and that the BCD has a default OS entry.
- Verify that the disk has the unoccupied space needed for GPT metadata and, if necessary, a new EFI partition.
- Use the logs to identify an unreadable volume ID or unrecognized partition type rather than guessing which partition to alter.
- Make a verified backup before changing or removing any partition.
Do not delete a recovery or vendor partition merely to meet the count unless you understand its purpose and have an independent recovery plan. MBR2GPT’s validation output and logs should determine which condition actually failed.
Where are the MBR2GPT logs, and which files matter most?
When validation or conversion fails, inspect the logs instead of relying on the short console message. MBR2GPT writes four diagnostic files: diagerr.xml, diagwrn.xml, setupact.log, and setuperr.log.
The default location is the Windows directory used by the running environment. You can choose another location with the /logs:<logDirectory> option. The exact path matters when you are working in Windows PE or WinRE, because that environment may use a different drive letter for the Windows volume.
setupact.log and setuperr.log contain the most detailed information about disk layout checks and processing. The XML files record diagnostic errors and warnings that can help correlate the console result with a specific partition or operation.
What to record before changing anything
- The complete command, including the disk number and any switches.
- Whether the command ran in Windows PE/WinRE or full Windows with
/allowFullOS. - The displayed return code.
- The contents of
setupact.logandsetuperr.logaround the first error. - The current partition style, partition count, active system partition, and BitLocker state.
Capturing this information prevents a later repair attempt from obscuring the original failure.
What do MBR2GPT return codes mean?
The return code narrows the repair to a phase. Code 0 means conversion completed; code 1 means the user canceled it.
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| Code | Meaning | First response |
|---|---|---|
| 2 | Internal error | Read the detailed logs and avoid repeated conversion attempts until the failing operation is identified. |
| 3 | Initialization error | Check the execution environment, target disk, and log path, then rerun validation. |
| 4 | Invalid command-line parameters | Correct the syntax, disk number, or unsupported switch. |
| 5 | Error reading disk geometry or layout | Verify that the disk is accessible and inspect the layout-related log entries. |
| 6 | One or more volumes are encrypted | Suspend BitLocker protection before retrying. |
| 7 | Disk geometry or layout does not meet requirements | Fix partition count, extended/logical partitions, active-system status, metadata space, BCD, volume IDs, or partition types. |
| 8 | EFI System Partition creation failed | Check available space and the execution environment; inspect the EFI creation entries in the logs. |
| 9 | Boot-file installation failed | Check the system partition and BCD conditions, then retry from a supported environment. |
| 10 | GPT layout application failed | Stop and review disk-write, geometry, and metadata details in the logs before another attempt. |
| 100 | Conversion succeeded, but some BCD entries were not restored | Treat the disk as converted and repair startup configuration through Windows recovery tools. |
Codes 6 through 10 identify the phase that matters most when a message says only “failed.” A code 100 result is different from a rejected conversion: the partition-style conversion completed, but startup configuration needs attention.
How do I fix “mbr2gpt failed” with BitLocker or another encryption state?
Microsoft allows conversion of BitLocker-encrypted volumes when BitLocker protection is suspended. A locked or actively protected volume can produce return code 6.
- Confirm which Windows volumes are protected and ensure you have the recovery information required by your organization.
- Suspend BitLocker protection using the normal Windows or management controls for that installation.
- Run
mbr2gpt /validateagain and resolve any independent layout errors. - Convert only after validation succeeds.
- After conversion and the firmware change, follow Microsoft’s documented post-conversion procedure for recreating or resuming protectors.
Suspending protection is not the same as decrypting the volume. Do not assume that a successful validation means protectors have already been restored; check the encryption state after Windows boots in UEFI mode.
How do I fix “mbr2gpt failed to create EFI system partition”?
This message corresponds to return code 8. MBR2GPT could not create or prepare the EFI System Partition required for UEFI booting.
Check space and partition placement
Review setupact.log and setuperr.log for the reason the EFI operation stopped. Confirm the documented free-space requirements for GPT metadata and the required EFI partition size: 100 MB on ordinary-sector disks or 260 MB on 4K-sector disks when a new EFI partition is needed.
Check the execution environment
In Windows PE, the existing system partition is not being used by the running Windows installation, so MBR2GPT has more flexibility. With /allowFullOS, the system partition is in use and the EFI-creation method changes. If a full-OS attempt reaches code 8, repeat the diagnostic workflow from Windows PE/WinRE rather than repeatedly forcing the same in-use layout.
Do not improvise partition deletion
An EFI failure does not prove that a particular recovery or vendor partition is disposable. Identify the exact blocked operation in the logs, preserve a backup, and make only a layout change that addresses that documented condition.
How do I fix “mbr2gpt failed to install boot files”?
Return code 9 means the conversion reached the boot-file installation phase but could not place or configure the UEFI boot files.
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- Confirm that the system partition’s BCD contains a valid default Windows entry.
- Check the logs for an unreadable volume ID, an unexpected partition type, or an EFI partition operation that failed immediately before boot-file installation.
- If the attempt ran in the full OS, use Windows PE/WinRE for the next validation and conversion attempt when practical.
Do not treat a code 9 result as proof that GPT metadata was applied successfully. Follow the phase reported by the tool; later conversion steps may never have run.
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How do I fix an MBR2GPT GPT-layout or geometry failure?
Return code 7 indicates that the disk failed the documented geometry or layout requirements during validation. Return code 10 indicates that MBR2GPT reached GPT-layout application but could not complete it.
For code 7
Recheck the primary-partition count, extended or logical partitions, active system partition, BCD default entry, readable volume IDs, recognized partition types, and unoccupied metadata space. These are eligibility failures, so conversion should not be attempted until validation passes.
For code 10
Read the log entries immediately before the GPT-layout error. Confirm that the target disk is still accessible and that the geometry and metadata areas described by the tool are available. A second conversion attempt without correcting the logged condition can make it harder to distinguish the original failure from a new one.
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Code 100 means the disk conversion succeeded but some BCD entries were not restored. The partition style is therefore different even though Windows may not start normally.
First switch the firmware to UEFI, because a converted GPT system disk requires UEFI booting. If Windows still does not start, use Windows Recovery Environment and choose Troubleshoot → Advanced options → Startup Repair. Microsoft says Startup Repair can diagnose and attempt repairs for damaged system files, a corrupted BCD, a damaged MBR, incompatible drivers, and registry errors. For a conversion-specific question, retain the original MBR2GPT logs; Startup Repair addresses startup recovery, not the cause of an earlier partition-validation failure.
How do I convert after validation succeeds?
Run conversion only after the validation command completes without errors and BitLocker protection has been suspended when encryption is present.
Conversion from Windows PE
mbr2gpt /convert /disk:<diskNumber>Conversion from the full Windows 10 environment
mbr2gpt /convert /disk:<diskNumber> /allowFullOSThe conversion command performs validation again before changing the disk. It then creates or reuses an EFI System Partition, installs UEFI boot files, applies GPT metadata, updates BCD, and restores drive-letter assignments. Watch the status output and keep the logs available if any step reports an error.
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MBR2GPT changes the disk to GPT, but firmware settings are separate from the partition table. After conversion, enter the device firmware setup and change the boot mode from legacy BIOS or CSM to UEFI.
Do this before judging the conversion as a startup failure. A GPT disk left in legacy boot mode may not start even when MBR2GPT completed correctly. The exact firmware menu name varies by manufacturer, so look for a boot-mode setting that distinguishes UEFI from Legacy or CSM.
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Should I run MBR2GPT from Windows or Windows PE?
Windows PE or WinRE is the normal environment and generally gives MBR2GPT the least interference from the installed system. Full Windows execution is supported with /allowFullOS, but the existing system partition is in use and EFI partition creation follows a different path.
| Consideration | Windows PE/WinRE | Full Windows with /allowFullOS |
|---|---|---|
| System partition in use | No, the installed OS is offline | Yes |
| Validation switch | /validate /disk:<diskNumber> |
/validate /disk:<diskNumber> /allowFullOS |
| Conversion switch | /convert /disk:<diskNumber> |
/convert /disk:<diskNumber> /allowFullOS |
| EFI creation behavior | Uses the offline system-partition context | Changes because the existing system partition is in use |
| Best use | Normal conversion workflow and recovery | When booting into PE is not practical and the layout supports full-OS execution |
The choice of environment does not remove the layout prerequisites. Both contexts still require a valid MBR system disk, acceptable partition structure, readable identifiers, and a usable BCD.
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What should I do if Windows will not boot after MBR2GPT?
- Return to firmware setup and confirm that boot mode is UEFI rather than Legacy or CSM.
- If BitLocker is enabled, be prepared to use the recovery information requested by Windows after the hardware or boot configuration change.
- Enter Windows Recovery Environment.
- Select Troubleshoot → Advanced options → Startup Repair.
- If Startup Repair does not resolve the problem, preserve the MBR2GPT logs and distinguish this startup problem from the original conversion phase.
Startup Repair is appropriate for post-conversion startup failures, including damaged boot configuration and system files. It is not a substitute for correcting a disk that still fails MBR2GPT validation.
How do I protect data before retrying?
Although Microsoft describes MBR2GPT as converting without modifying or deleting data, changing a partition layout is still a high-impact operation. Make a verified full backup or disk image before retrying, and ensure that the backup can be accessed independently of the Windows disk.
- Confirm that the backup includes the files you cannot replace and that it is readable from recovery media.
- Record the current disk number, partition map, firmware mode, and encryption state.
- Keep BitLocker recovery information available before suspending protection.
- Do not interrupt the conversion once it has started unless the tool explicitly reports an error or cancellation.
- Keep copies of the four MBR2GPT logs for troubleshooting or recovery support.
Which repair path should I choose?
| Observed result | Repair focus | Next safe action |
|---|---|---|
| Validation fails before conversion | Disk eligibility and partition layout | Correct the documented requirement, then validate again. |
| Code 6 | Encryption protection | Suspend BitLocker protection and rerun validation. |
| Code 8 | EFI System Partition creation | Inspect free space and environment; prefer PE/WinRE when full-OS creation is blocked. |
| Code 9 | UEFI boot-file installation | Check the system partition, BCD default entry, and preceding log errors. |
| Code 10 | GPT metadata application | Review geometry, metadata space, and disk-access errors before retrying. |
| Code 100 or no boot after code 0 | Post-conversion startup | Set firmware to UEFI and run Startup Repair from WinRE if needed. |
The reliable sequence is always the same: identify the Windows system disk, validate it, read the logs, repair the phase-specific condition, validate again, convert once, and then configure UEFI firmware.
Frequently Asked Questions
How do I identify the correct disk number when several disks are installed?
Match the disk that contains the Windows volume and its system partition in Disk Management, then cross-check the same capacity and partition arrangement in DiskPart or PowerShell. Record that number before running either command; selecting a different MBR disk can produce a misleading result or affect the wrong installation.
What information should I give a technician when MBR2GPT keeps failing?
Provide the exact command, target disk number, whether it ran in Windows PE or full Windows, the displayed return code, and the relevant sections of setupact.log and setuperr.log. Include the current partition style, partition count, firmware mode, and whether BitLocker protection was suspended.
What is the practical difference between a file backup and a disk image for this operation?
A file backup protects personal data, while a disk image captures the Windows installation and its boot-related partitions so the original layout can be restored more completely. Whichever method you choose, verify that the backup is readable independently of the disk being converted.
Why can a computer boot in Windows before conversion but not immediately afterward?
The conversion changes the partition style from MBR to GPT, while firmware may still be configured for Legacy or CSM booting. The converted disk requires UEFI mode, so a firmware change is part of completing the conversion rather than evidence that the files were deleted.
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