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If Windows stopped booting immediately after you changed firmware from Legacy BIOS or CSM to UEFI, first match the firmware mode to the system disk. A legacy installation is usually on an MBR disk and needs Legacy/CSM mode; a UEFI installation needs a GPT disk and an EFI System Partition. Changing the firmware setting does not convert MBR to GPT.
Restore the previous Legacy/CSM setting if the disk is still MBR and you need to get back into Windows. After backing up your files, use Microsoft’s MBR2GPT utility if the system disk passes validation, then configure firmware for UEFI and select Windows Boot Manager. If the disk is already GPT, leave UEFI enabled and repair the Windows boot chain from Windows Recovery Environment (WinRE), starting with Startup Repair.
What changed when you switched to UEFI
Legacy BIOS and UEFI use different boot paths. A traditional BIOS installation normally starts from boot code on an MBR-partitioned disk. A UEFI installation starts through an EFI System Partition (ESP), a FAT32 partition on a GPT disk, and a firmware boot entry such as Windows Boot Manager.
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Use this decision rule:
| What you find | Appropriate path | Important caution |
|---|---|---|
| The Windows disk is MBR and Windows was installed in Legacy/CSM mode | Restore Legacy/CSM temporarily, or convert the eligible system disk with MBR2GPT and then move to UEFI | Run validation first, suspend BitLocker, back up data, and reconfigure firmware after conversion. |
| The Windows disk is GPT and firmware is UEFI | Keep UEFI enabled; repair Windows Boot Manager, the ESP, or the BCD from WinRE | Do not apply BIOS/MBR repair commands without evidence that they are appropriate. |
| Firmware or WinRE cannot see the Windows disk | Investigate storage detection, cabling, controller mode, or hardware | Boot-repair commands cannot repair a disk that is not detected. Contact the PC manufacturer if it remains absent. |
Windows begins loading and then shows INACCESSIBLE_BOOT_DEVICE |
Check storage-controller mode, drivers, and boot-partition files | This can be a controller or driver problem rather than a UEFI/MBR mismatch. |
1. Find out where startup stops
Disconnect peripherals and observe the stage of failure
Remove external USB drives, memory cards, docks, and other nonessential devices, then start the PC. If it never gets past the manufacturer logo or firmware screen, the problem may be hardware or storage detection. Microsoft’s startup guidance treats a system stuck in the BIOS phase differently from one that reaches the Windows boot loader.
A missing or corrupt BCD message, a missing boot manager message, or a blinking cursor usually means firmware has completed its work and the failure is in the Windows boot-loader stage. An immediate return to firmware setup can also mean that no bootable UEFI entry or loader was found.
Check whether firmware sees the drive
Open the firmware setup menu and look for the SSD or hard drive in storage information. The exact menu and labels vary by manufacturer. If the drive is not listed there, stop changing Windows boot files: Windows commands cannot repair a device that firmware cannot detect. Check power and data connections in a desktop, remove and reseat a removable drive if the manufacturer permits it, and review storage-controller settings. If the drive remains absent, use the PC or motherboard maker’s hardware support.
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Check visibility from WinRE
If the computer can reach recovery options, open Troubleshoot > Advanced options > Command Prompt. At the prompt, run:
- Type
diskpartand press Enter. - Run
list diskto see whether the physical Windows disk is present. - Run
list volto see Windows and any small FAT32 system volume. - Type
exitto leave DiskPart.
WinRE can assign different drive letters than normal Windows, so do not assume the installation is on C:. If the OS disk does not appear in list disk, stop applying boot-file commands and investigate firmware, the storage controller, the connection, or the drive itself. Microsoft advises contacting the OEM or virtualization provider when the OS disk is not visible.
2. Confirm whether the disk is MBR or GPT
In DiskPart, list disk shows an asterisk in the GPT column for a GPT disk. If the asterisk is absent, the disk is generally MBR. You can also select the disk and inspect it:
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diskpart
list disk
select disk 0
detail disk
Do not blindly use select disk 0 on a system with multiple drives. Identify the disk by its size and model before selecting it. A wrong disk number can lead to destructive commands later, particularly if someone follows generic DiskPart instructions.
The partition style determines the next step:
- MBR: An existing Windows installation normally needs Legacy/CSM mode until it is converted.
- GPT: Keep UEFI enabled and investigate Windows Boot Manager, the ESP, and BCD configuration.
Microsoft’s Windows Setup partition guidance explains the relationship between firmware mode and partition style. Reformatting during Setup is not a repair step: deleting or formatting partitions erases data.
3. If the disk is MBR: recover first, then decide whether to convert
Temporarily restore Legacy/CSM mode
If the installation was working before you changed the firmware setting and the disk is MBR, enter firmware setup and restore the previous Legacy, BIOS, or CSM option. Save the change and try Windows again. This is a recovery and diagnostic step, not a conversion; it lets you boot the existing installation so you can back up files and prepare a controlled migration.
Some newer systems no longer offer CSM. If the firmware has no compatible Legacy option, use installation media to enter WinRE and assess whether the disk is eligible for MBR2GPT. Confirm that the machine supports UEFI and note the manufacturer’s terminology for UEFI-only boot, CSM, and Secure Boot. Firmware menus and post-conversion behavior differ by model.
Back up before modifying partitions
Copy important files to another disk or a trusted backup location before conversion, partition edits, or reinstalling Windows. Microsoft documents MBR2GPT as a conversion that does not delete user data when it succeeds, but it can reject an ineligible layout and no partition operation is risk-free. Do not treat a successful validation as a replacement for a backup.
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Check MBR2GPT eligibility
MBR2GPT.EXE is included with supported Windows installations. It can run from Windows PE or from the full operating system with /allowFullOS. It converts an MBR system disk; it is not a general-purpose converter for a non-system data disk.
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Microsoft’s documented eligibility rules include all of the following:
- The target disk is MBR.
- There are no more than three primary partitions.
- The disk has an active system partition.
- There are no extended or logical partitions.
- The BCD contains a valid default operating-system entry.
- Partition types are recognized and volume identifiers can be retrieved.
- There is enough usable space for GPT structures and the new ESP.
BitLocker needs additional preparation. Suspend protection before conversion. Microsoft’s documentation says existing protectors must be deleted and recreated before protection is resumed after the disk has been converted. Keep the BitLocker recovery key available; WinRE may request it.
Validate before converting
From an elevated Windows Command Prompt, or from WinPE, use the documented syntax with the verified disk number:
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MBR2GPT /validate /disk:<diskNumber> /logs:<logDirectory>If you are running inside the full Windows installation rather than WinPE, add /allowFullOS:
MBR2GPT /validate /disk:<diskNumber> /allowFullOSValidation checks the layout without converting it. If it fails, read the log and correct the specific eligibility problem; do not force the operation with destructive partition commands. Common blockers include four or more primary partitions, an extended/logical partition, an invalid BCD entry, or insufficient room for the required structures.
Convert only after validation passes
After confirming the disk number and making a backup, run the corresponding conversion command:
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MBR2GPT /convert /disk:<diskNumber> /allowFullOSWhen running from WinPE, omit /allowFullOS if it is not needed. The utility validates again before changing the disk. If conversion completes, Microsoft’s instruction is explicit: “After the disk has been converted to GPT partition style, the firmware must be reconfigured to boot in UEFI mode.”
Reconfigure firmware after conversion
Restart into firmware setup and:
- Disable Legacy/CSM boot if the system requires UEFI-only mode.
- Enable UEFI boot.
- Place Windows Boot Manager for the converted disk ahead of other boot options.
- Leave storage-controller settings unchanged unless you are deliberately correcting a known controller problem.
Save and reboot. If Windows still does not start, return to WinRE and use the GPT/UEFI repair path below. Do not convert the disk repeatedly or switch modes randomly; record each firmware change so you can undo it.
4. If the disk is already GPT: repair the UEFI boot chain
Enter Windows Recovery Environment
If WinRE appears automatically, choose Troubleshoot > Advanced options. If it does not, create Windows installation media on an ordinary compatible USB flash drive using another computer, boot the affected PC from that media, and choose Repair your computer rather than installing Windows. The USB drive only provides access to recovery tools; it does not convert the disk or repair the ESP by itself.
Run Startup Repair first
Select Troubleshoot > Advanced options > Startup Repair. Microsoft says Startup Repair can diagnose and repair common startup problems, including damaged system files and boot-configuration errors. It applies to Windows 10 and Windows 11. If the device is encrypted, have the BitLocker recovery key ready; Microsoft states that the key may be required to complete Startup Repair.
Allow the process to finish and restart once. If it reports that it could not repair the PC, return to Advanced options rather than repeatedly running it without checking the disk and partitions.
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Open Command Prompt and use DiskPart to inspect volumes:
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diskpart
list disk
list vol
Look for the Windows volume and a small FAT32 volume that is the ESP. Drive letters in recovery are temporary. Use directory listings such as dir C:, dir D:, and so on to find the volume containing the Windows directory. Assign a temporary letter to the ESP only after you have identified it; avoid formatting it.
In a UEFI installation, Microsoft identifies the Windows Boot Manager path as EFIMicrosoftBootbootmgfw.efi. The loader files use the .efi format. You can inspect the boot configuration with:
bcdedit /enum allCheck that the Windows Boot Manager and Windows loader entries point to the intended disk and installation. If the ESP is missing, unreadable, or the BCD entries are damaged, the repair must rebuild the UEFI boot files or BCD on the ESP. Follow the exact volume and Windows-path information you established in WinRE; do not assume normal Windows drive letters.
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Microsoft’s general startup troubleshooting documentation lists these commands for specific boot-code and BCD situations:
BOOTREC /FIXMBR
BOOTREC /FIXBOOT
BOOTREC /SCanos
It also documents a BCD rebuild sequence for corresponding BCD failures. These are not universal UEFI remedies. /FIXMBR repairs master boot code; it does not correct every partition-table problem, and it does not by itself recreate missing UEFI boot files on the ESP. Do not run BIOS/MBR commands blindly on a GPT/UEFI installation. First establish whether the symptom is boot-code damage, a BCD problem, or missing EFI files.
5. When the message is “INACCESSIBLE_BOOT_DEVICE”
If the Windows logo appears and startup then stops with INACCESSIBLE_BOOT_DEVICE, the firmware may have found the loader successfully. Investigate storage-controller mode and drivers as well as partition corruption.
- Check whether SATA, NVMe, RAID, AHCI, or other controller settings changed when you altered firmware options.
- Restore the controller mode that Windows used when it was installed, if you can identify it safely.
- Consider a changed controller, missing storage driver, hardware fault, or corrupted boot-partition files.
- Use WinRE to verify that the OS disk and Windows volume are visible before attempting boot repairs.
Do not equate this stop error automatically with MBR versus GPT. Microsoft lists controller changes, different hardware controllers, hardware faults, and corrupted boot-partition files among possible causes.
6. Common mistakes that make recovery harder
- Formatting or deleting partitions first: Windows Setup warns that reformatting erases data. Use it only when you have a verified backup and have decided to perform a clean installation.
- Running MBR2GPT on the wrong disk: Verify the disk by model and capacity; never guess a disk number in a multi-drive computer.
- Skipping validation: MBR2GPT is designed to reject unsupported layouts. A failed validation is a reason to inspect the layout, not to force conversion.
- Applying legacy Bootrec steps to every UEFI failure: GPT/UEFI systems may need ESP and BCD repair instead of master-boot-code changes.
- Assuming the Windows partition is C: in WinRE: Find the volume containing the Windows directory before inspecting BCD or running repairs.
- Ignoring BitLocker: Suspend protection before conversion and keep the recovery key available for WinRE and Startup Repair.
- Continuing Windows repairs when the disk is undetected: If firmware or
list diskcannot see the device, troubleshoot hardware, connections, and controller settings first. - Changing several firmware settings at once: Record the original boot mode, controller mode, Secure Boot state, and boot order so you can reverse one change at a time.
7. A practical recovery checklist
- Disconnect external boot devices and determine whether the PC reaches firmware, the Windows loader, or the Windows logo.
- Confirm that firmware lists the Windows drive.
- Open WinRE, if available, and use DiskPart
list diskandlist volto confirm visibility. - Determine whether the system disk is MBR or GPT.
- If it is MBR, restore Legacy/CSM temporarily if possible, back up data, and validate MBR2GPT before conversion.
- If it is GPT, leave UEFI enabled and run Startup Repair.
- If Startup Repair fails, inspect volume letters, the FAT32 ESP, and BCD entries before choosing manual commands.
- If the disk is absent from firmware or WinRE, stop boot-file repairs and obtain hardware or manufacturer support.
How to avoid the problem on a future firmware change
Before enabling UEFI on an existing Windows installation, identify the system disk’s partition style and make a current backup. If it is MBR, verify that the computer supports UEFI and that the disk meets MBR2GPT’s layout requirements. Suspend BitLocker, run MBR2GPT /validate, and plan the firmware change immediately after a successful conversion. Afterward, select Windows Boot Manager and confirm that Windows starts before changing unrelated settings such as Secure Boot or storage-controller mode.
For a clean installation, choose the firmware mode first and let Windows Setup create the matching partition layout. Do not switch between UEFI and Legacy/CSM casually on an installed system: each mode expects a different boot path.
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