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 and boot configuration on the disk it was asked to validate. First confirm that Disk 0 is the intended Windows system disk, then check where its boot files live and review the validation logs before changing partitions.
What the error means
MBR2GPT converts an eligible MBR system disk to GPT. Before converting, 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 text alone does not tell you which check failed or establish that a partition is absent. See Microsoft’s MBR2GPT documentation for the tool’s requirements and behavior.
As an Amazon Associate I earn from qualifying purchases.
Check which disk Windows actually boots from
Confirm the target disk number
Disk numbers can vary between Windows, Windows PE, and other boot environments. Do not assume Disk 0 is the Windows disk just because it has that number in another session. In DiskPart, inspect list disk and list volume; Disk Management can also help map volumes and partitions to physical disks. Microsoft’s MBR2GPT command-line option /disk:<diskNumber> selects a disk explicitly; if you omit it, the tool selects the system disk.
Free tools Windows power users keep installed
One-click scans. No signup required.
Map the Windows volume and boot partition
Compare the disk containing the Windows installation with the disk containing the system or boot partition. Inspect DiskPart output and Disk Management, and use bcdedit to examine the boot configuration. A Microsoft Q&A report for this exact error described Windows on Disk 0 while the System Reserved partition containing boot files was on Disk 1. Microsoft External Staff identified that split layout as the likely cause in that case—not as a universal explanation. The case is documented in the Microsoft Q&A thread.
#1 Best Overall
- Dual USB-A & USB-C Bootable Drive – compatible with nearly all Windows PCs, laptops, and tablets (UEFI & Legacy BIOS). Works with Surface devices and all major brands.
- Fully Customizable USB – easily Add, Replace, or Upgrade any compatible bootable ISO app, installer, or utility (clear step-by-step instructions included).
- Complete Windows Repair Toolkit – includes tools to remove viruses, reset passwords, recover lost files, and fix boot errors like BOOTMGR or NTLDR missing.
- Reinstall or Upgrade Windows – perform a clean reinstall of Windows 7 (32bit and 64bit), 10, or 11 (amd64 + arm64) to restore performance and stability. (Windows license not included.). Includes Full Driver Pack – ensures hardware compatibility after installation. Automatically detects and installs drivers for most PCs.
- Premium Hardware & Reliable Support – built with high-quality flash chips for speed and longevity. TECH STORE ON provides responsive customer support within 24 hours.
Check MBR2GPT’s validation requirements
For the selected disk to pass validation, Microsoft specifies these layout and boot prerequisites:
- The disk must use MBR partitioning and be a system disk; MBR2GPT cannot convert a non-system disk.
- It must have no more than three primary partitions and no extended or logical partitions.
- It must have an active system partition.
- The BCD store’s default OS entry must point to an OS partition.
- Volume IDs must be retrievable for volumes that have drive letters.
- Partition types must be recognized by Windows or explicitly mapped.
A failure involving any of these conditions can prevent validation. Use the logs to identify the specific failure rather than treating the displayed OS-partition message as a complete diagnosis.
Run validation and inspect the logs
Validate before converting
Run MBR2GPT with /validate first. Validation checks eligibility without converting the disk. MBR2GPT is available in Windows PE and full Windows; when running it from full Windows, Microsoft requires the /allowFullOS option. Select the intended disk explicitly when appropriate, for example: mbr2gpt /validate /disk:0 /allowFullOS. Use that command only if Disk 0 is confirmed as the target and you are running from full Windows. Do not run /convert until validation passes and you understand the disk and boot layout.
Quick wins for a faster PC:
Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Use MBR2GPT’s diagnostic files
MBR2GPT writes diagerr.xml, diagwrn.xml, setupact.log, and setuperr.log. Microsoft identifies setupact.log and setuperr.log as the most detailed sources for disk layout, tool activity, and validation results. Check these files for the failed condition before attempting boot or partition repairs.
Choose the execution environment with the boot layout in mind
Microsoft supports running MBR2GPT from either Windows PE or full Windows, but the existing system partition affects how the EFI System Partition is created:
Rank #2
- High-speed USB 3.0 performance of up to 150MB/s(1) [(1) Write to drive up to 15x faster than standard USB 2.0 drives (4MB/s); varies by drive capacity. Up to 150MB/s read speed. USB 3.0 port required. Based on internal testing; performance may be lower depending on host device, usage conditions, and other factors; 1MB=1,000,000 bytes]
- Transfer a full-length movie in less than 30 seconds(2) [(2) Based on 1.2GB MPEG-4 video transfer with USB 3.0 host device. Results may vary based on host device, file attributes and other factors]
- Transfer to drive up to 15 times faster than standard USB 2.0 drives(1)
- Sleek, durable metal casing
- Easy-to-use password protection for your private files(3) [(3)Password protection uses 128-bit AES encryption and is supported by Windows 7, Windows 8, Windows 10, and Mac OS X v10.9 plus; Software download required for Mac, visit the SanDisk SecureAccess support page]
| Environment | What to know |
|---|---|
| Full Windows | Use /allowFullOS. The existing MBR system partition is in use and cannot be reused as the EFI System Partition, so MBR2GPT creates a new EFI partition by shrinking the OS partition. |
| Windows PE | The existing MBR system partition can be reused if it meets Microsoft’s documented conditions. |
Choose an environment you can recover from and verify the target disk and boot arrangement in that environment before proceeding.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Understand the risks and what happens after conversion
Microsoft describes MBR2GPT as converting the disk without modifying or deleting data, but manual partition or boot-configuration changes still carry risk. Back up important files and make sure you have a recovery route before making changes. Microsoft notes that drive-letter remapping occurs after layout conversion and cannot be undone at that stage. It also documents return code 100 as meaning GPT conversion succeeded even though some boot configuration entries could not be restored.
Windows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallOutdated Drivers Are Slowing You Down
One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchAfter a successful conversion, the device must support UEFI and its firmware must be configured to boot in UEFI mode. Confirm both before converting; a GPT disk alone does not ensure the machine will boot successfully.
When to investigate BCD or boot-mode issues
If the target disk and partition layout appear eligible, examine whether the BCD entries point to the correct devices and whether the machine’s actual boot mode matches the configuration you are inspecting. A secondary technical article discusses incorrect BCD references and differences involving the expected BootBCD path and UEFI boot paths: Partition Wizard’s MBR2GPT troubleshooting article. Treat that as secondary guidance, not proof that a particular BCD repair applies to your PC. Confirm the boot mode and the failed validation check before changing boot files or running repair commands.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
