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The message “Cannot find OS partition(s) for disk 0” does not prove that Windows is missing. It means MBR2GPT could not identify the expected operating-system partition on the disk it was asked to validate. First verify that Disk 0 is the intended system disk, then check where Windows and its boot files are located, review the conversion requirements, and read the MBR2GPT logs. Do not run the conversion until validation succeeds and you understand the machine’s boot layout.
What the error means
MBR2GPT converts an eligible MBR system disk to GPT. During validation, it checks the disk layout and boot configuration, including whether the BCD store’s default operating-system entry points to an OS partition. The error is a symptom, not a complete diagnosis: it does not by itself establish that a partition is absent or identify which prerequisite failed. See Microsoft’s MBR2GPT documentation for the tool’s requirements and behavior.
One plausible cause is a split boot layout: Windows may be on Disk 0 while the system or boot partition is on another disk. In a Microsoft Q&A case reporting this exact error, a Microsoft External Staff moderator identified that arrangement as the likely cause for that computer. Treat it as a hypothesis to check, not a universal explanation; the case-specific discussion does not establish that every machine showing the message has the same layout.
Check the selected disk and boot layout
- Confirm what Disk 0 refers to. In an elevated Command Prompt, run
diskpart, thenlist diskandlist volume. You can also inspect the disks and partitions in Disk Management. Disk numbers can differ between environments, so do not assume Disk 0 is the Windows system disk without checking. MBR2GPT’s/disk:<diskNumber>option selects a disk; if you omit it, the tool selects the system disk. - Locate Windows and the boot partition. Review Disk Management and the DiskPart output, then inspect the current boot configuration with
bcdedit. Identify the Windows volume and the partition containing the system boot files, and note which physical disk holds each. If they are on different disks, record that before proceeding; it may explain why validation cannot associate the expected OS and boot configuration with the selected disk. - Check the full set of MBR2GPT requirements. Microsoft requires the selected disk to be an MBR system disk with no more than three primary partitions, no extended or logical partitions, an active system partition, and a BCD default OS entry that points to an OS partition. Volume IDs for lettered volumes must be retrievable, and partition types must be recognized by Windows or explicitly mapped. Review the official requirements before making layout changes.
- Validate the explicitly identified disk. Run validation before conversion. From full Windows, include
/allowFullOS; MBR2GPT is also available in Windows PE, where that option is not needed. For example, from an elevated Command Prompt in full Windows, usembr2gpt /validate /disk:0 /allowFullOSonly after confirming that disk 0 is the intended target. A successful validation establishes that the tool’s checks passed for that selected disk; it does not replace the need to confirm your firmware supports UEFI and that you can recover the system.
Use the logs to identify the failed check
MBR2GPT creates diagerr.xml, diagwrn.xml, setupact.log, and setuperr.log. Microsoft identifies setupact.log and setuperr.log as the most detailed records of the disk layout, validation steps, and conversion process. Read them alongside the disk and BCD information rather than treating the one-line error as a diagnosis. The log paths and additional command details are described in Microsoft’s MBR2GPT documentation.
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If the logs point to the BCD store, check the actual boot mode and the device entries before attempting boot repairs. Incorrect or stale BCD entries are discussed as a possible explanation in this secondary technical reference: EaseUS’ MBR2GPT troubleshooting article. That advice is not a substitute for verifying the machine’s configuration, and the error alone is not a reason to run repair commands.
Choose Windows PE or full Windows deliberately
Microsoft supports running MBR2GPT from either Windows PE or full Windows. The important difference is how the existing system partition can be used during conversion:
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| Execution context | Option needed | System-partition behavior |
|---|---|---|
| Full Windows | /allowFullOS |
The existing MBR system partition is in use and cannot be reused as the EFI System Partition. MBR2GPT creates a new EFI partition by shrinking the OS partition. |
| Windows PE | Not stated as required by Microsoft’s documentation | The existing MBR system partition can be reused if it meets the documented conditions. |
Choose the environment based on your available recovery access and the machine’s boot layout, not as a workaround for a failed validation. In either environment, specify and validate the intended disk before converting.
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Before converting, understand the boot and recovery consequences
- UEFI is required after a successful conversion. The computer’s firmware must be configured to boot in UEFI mode, and the device must support UEFI. Confirm this before changing the disk.
- Back up important data and confirm a recovery route. Microsoft describes MBR2GPT as converting without modifying or deleting data, but manual partition or boot-configuration changes still carry risk.
- Conversion is not a simple undo. Microsoft notes that changes cannot be undone once drive-letter remapping occurs after layout conversion.
- Interpret return code 100 carefully. Microsoft says this return code means GPT conversion succeeded even though some boot configuration entries could not be restored. It therefore does not mean the disk conversion itself failed; check boot configuration and recovery options before relying on the converted system.
When to stop and investigate further
Do not proceed to /convert if validation still fails, if you cannot tell which disk contains Windows and the boot files, or if the intended disk does not meet the documented requirements. Use the MBR2GPT logs to find the specific failure and resolve the underlying layout or configuration issue before trying again. Microsoft documents that /validate checks eligibility without converting, while /convert validates and then converts only if the checks pass.
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