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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsIf Windows stopped booting immediately after you changed Legacy BIOS or CSM to UEFI, first match the firmware mode to the system disk’s partition style. A legacy installation is usually on an MBR disk and needs Legacy/CSM until it is converted. A UEFI installation needs a GPT disk and an EFI System Partition (ESP). Changing the firmware setting alone does not convert MBR to GPT.
Check whether the firmware still detects the Windows drive, then identify the disk style in Windows Recovery Environment (WinRE). If the disk is MBR, temporarily restore Legacy/CSM or convert the eligible system disk with Microsoft’s MBR2GPT tool before selecting UEFI. If the disk is already GPT, leave UEFI enabled and repair the Windows Boot Manager, ESP, or boot configuration. A drive that is absent from firmware or WinRE needs storage or hardware diagnosis, not more boot-file commands.
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Why changing Legacy BIOS to UEFI can stop Windows from booting
Legacy BIOS and UEFI are different startup paths. Traditional BIOS installations normally boot from an MBR-partitioned disk. UEFI Windows installations boot from GPT and use a small FAT32 EFI System Partition containing the UEFI boot files. The firmware setting and the disk’s partition style therefore have to agree.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11When you switch only the firmware toggle, an existing MBR Windows installation is still MBR. UEFI may not offer it as a bootable device, or it may show a blank boot list or a “Windows Boot Manager missing” message. Conversely, changing a GPT installation to Legacy/CSM can hide its UEFI boot entry.
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Microsoft’s partition guidance describes the relationship between Windows Setup and MBR or GPT; the UEFI/GPT deployment guidance explains the ESP layout and sizing requirements: Windows Setup: Installing using the MBR or GPT partition style and UEFI/GPT-based hard drive partitions.
Do not format the disk as a first reaction. Windows Setup warns that reformatting removes data. MBR2GPT is designed to convert an eligible system disk without deleting its data, but it validates the layout first and no partition operation should be attempted without a backup.
Identify where startup stops before changing anything else
1. Disconnect external devices and observe the stage of failure
Shut the PC down, remove external drives, memory cards, docks, and other USB devices, and start it again. A machine that never gets past the manufacturer logo or firmware screen may have a hardware, storage-controller, or firmware configuration problem. A message about a missing boot manager, a damaged BCD, or a blinking cursor generally means firmware ran but could not continue into the Windows boot loader.
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Write down the exact message. “Windows Boot Manager missing” points toward a UEFI boot entry or ESP problem; INACCESSIBLE_BOOT_DEVICE appears after Windows begins loading and can instead indicate a storage-controller mode or driver issue.
2. Confirm that firmware sees the Windows drive
Enter the firmware setup using the key shown by the manufacturer, commonly F2, Delete, Esc, or F10. The label and menu location vary by PC maker. Look in storage information and the boot-device list for the internal SSD or hard drive.
- If the drive is listed, continue with the partition-style checks below.
- If it is not listed, power off and check the drive’s physical connection where accessible, the storage-controller setting, and whether another firmware storage mode is required. A Windows command cannot repair a device that firmware cannot detect.
- If the drive is visible in firmware but unavailable in WinRE, stop boot-file repairs and follow the manufacturer’s storage or virtualization support process.
3. Enter Windows Recovery Environment
If Windows presents recovery options, choose Troubleshoot > Advanced options. If recovery does not appear, start from Windows installation media and select Repair your computer instead of installing Windows. Microsoft’s startup troubleshooting guidance uses this same recovery route.
For encrypted devices, have the BitLocker recovery key available. Microsoft Support states: “If you’ve encrypted your device, you’ll need your BitLocker key to complete this task.”
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Check whether the system disk is MBR or GPT
From WinRE, open Troubleshoot > Advanced options > Command Prompt. Drive letters can change in recovery, so do not assume that Windows is C:.
- Start DiskPart:
diskpart- List physical disks:
list diskIn DiskPart, a marker in the GPT column identifies a GPT disk. A blank GPT column indicates MBR. Note the disk number that contains the Windows installation; do not guess it from the number alone if more than one disk is installed.
- List volumes and inspect the result:
list volLook for the Windows volume and, on a UEFI installation, a small FAT32 EFI System Partition. Record the volume letters shown in recovery. Then leave DiskPart:
exitIf list disk does not show the expected OS disk, do not run boot-repair commands. The problem is detection, connection, controller configuration, or hardware rather than a missing BCD entry.
Choose the repair path that matches the disk
| Situation | Appropriate path | Key caution |
|---|---|---|
| Disk is MBR and Windows was installed in Legacy BIOS mode | Restore Legacy/CSM temporarily, or convert the eligible system disk with MBR2GPT and then use UEFI | Validate first, suspend BitLocker, back up data, and change firmware only after a successful conversion |
| Disk is GPT and firmware is UEFI | Keep UEFI enabled; repair Windows Boot Manager, the ESP, or BCD, beginning with Startup Repair | Do not apply legacy MBR repairs without evidence of an MBR boot-code problem |
| Firmware or WinRE does not detect the OS disk | Investigate the drive, connection, storage-controller mode, or hardware and contact the PC maker if it remains absent | Bootrec cannot repair an undetected drive |
Windows starts loading and then shows INACCESSIBLE_BOOT_DEVICE |
Check storage-controller mode, controller drivers, and boot-partition files | This symptom may not be caused by a Legacy-versus-UEFI mismatch |
If the disk is MBR: recover first, then decide whether to convert
Temporarily restore Legacy or CSM
If the firmware still offers the previous mode, switch back to Legacy BIOS or CSM and make the internal Windows disk the first boot device. This should restore the existing MBR installation in many cases and gives you an opportunity to back up files. It is a recovery and diagnostic step, not a conversion to UEFI.
Firmware menus differ. Look for settings named Boot mode, UEFI/Legacy, CSM, or Compatibility Support Module. Save the change, restart, and confirm that Windows starts. If it does, copy important files to separate storage before editing partitions.
Check MBR2GPT eligibility
Microsoft’s MBR2GPT documentation describes a built-in utility that converts an MBR system disk to GPT. It can run from Windows PE or from the full operating system with /allowFullOS; it does not convert an arbitrary non-system data disk.
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Among the documented validation requirements, the target disk must:
- Be MBR-formatted and have room for the GPT structures.
- Contain no extended or logical partitions.
- Have no more than three primary partitions.
- Include an active system partition.
- Contain a valid BCD default operating-system entry.
- Have retrievable volume IDs and partition types that MBR2GPT recognizes.
If any requirement fails, the utility should be allowed to stop. Do not delete partitions merely to force validation to pass; that can destroy the installation.
Suspend BitLocker and secure the recovery key
Before conversion, make sure you can retrieve the BitLocker recovery key. Microsoft’s conversion guidance requires protection to be suspended; its documentation also says existing protectors need to be deleted and recreated to resume protection after the disk is converted. The exact BitLocker controls depend on how the device is managed, so record the recovery information before proceeding.
Validate the correct disk
Run the commands from an elevated Command Prompt in Windows or WinPE. Verify the disk number with DiskPart first. The documented syntax is:
MBR2GPT /validate /disk:<diskNumber>Replace <diskNumber> with the confirmed system-disk number. Validation checks the layout without converting it. If you are running from the full Windows installation rather than WinPE, include:
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Convert only after validation succeeds
Back up files and close applications before starting. The conversion command is:
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MBR2GPT /convert /disk:<diskNumber>From the full operating system, use:
MBR2GPT /convert /disk:<diskNumber> /allowFullOSMBR2GPT validates again and creates the GPT structures and UEFI boot files when the layout meets its requirements. If conversion reports an error, preserve the logs, return to the previous working firmware mode if necessary, and resolve the specific eligibility issue rather than experimenting with destructive DiskPart commands.
Reconfigure firmware after conversion
Conversion does not by itself change the firmware mode. Microsoft states: “After the disk has been converted to GPT partition style, the firmware must be reconfigured to boot in UEFI mode.” Return to setup, disable Legacy/CSM, enable UEFI, and select Windows Boot Manager for the converted disk. Manufacturer menus may also expose Secure Boot; follow the PC maker’s documented defaults rather than changing unrelated settings at the same time.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.If the disk is already GPT and UEFI is enabled
Run Startup Repair first
For a GPT disk in UEFI mode, leave the firmware configuration alone and start with the built-in repair. In WinRE, select Troubleshoot > Advanced options > Startup Repair. If WinRE is unavailable, boot Windows installation media and choose Repair your computer, then follow the same path.
Startup Repair can address common damaged system-file and boot-configuration problems. It may request the BitLocker recovery key. Let it complete and restart once; do not interrupt it by powering off.
Confirm the ESP and Windows volume from Command Prompt
If Startup Repair fails, return to Advanced options > Command Prompt. Use diskpart, list disk, and list vol to confirm that the GPT disk, the FAT32 ESP, and the Windows volume are visible. Recovery can assign different letters, so identify the Windows directory by checking candidate volumes rather than assuming C:.
In a UEFI installation, the Windows Boot Manager file is normally EFIMicrosoftBootbootmgfw.efi. Microsoft’s startup guidance uses bcdedit to inspect the boot configuration. Open it and verify that the entries point to the Windows installation and the EFI loader rather than to a missing volume:
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bcdeditIf the ESP is absent, unreadable, or not mounted, the boot files may need to be recreated on that partition. Use the exact recovery procedure appropriate to the Windows version and volume letters shown on your machine; do not assign a drive letter or overwrite an ESP until you have confirmed that it belongs to this installation.
Use Bootrec only for the failure it addresses
Microsoft’s general startup troubleshooting documentation lists these commands for specific boot-code and configuration cases:
BOOTREC /FIXMBR
BOOTREC /FIXBOOT
Bootrec /ScanOSIt also documents a BCD rebuild sequence for corresponding BCD failures. These commands are not a universal response to every UEFI problem. /FIXMBR repairs master boot code; it does not convert an MBR partition table to GPT, and it does not repair every ESP or UEFI boot-entry problem. On a GPT/UEFI installation, first establish whether the ESP and UEFI BCD are the damaged components. Running legacy-oriented commands blindly can add confusion without restoring the UEFI loader.
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When the message is “INACCESSIBLE_BOOT_DEVICE”
If the Windows logo appears and the system then stops with INACCESSIBLE_BOOT_DEVICE, investigate storage access as well as boot files. Microsoft lists a changed BIOS storage-controller setting, a different controller, hardware faults, and corrupted boot-partition files among possible causes.
- Return to firmware and compare the storage-controller mode with the setting used before the failure. A change such as AHCI to RAID, or the reverse, can prevent Windows from loading its expected driver.
- Confirm that the same physical disk is detected and that no controller or virtualization setting changed with the UEFI switch.
- In WinRE, use
diskpartandlist diskto verify that the OS disk is readable. - If the disk is readable and the controller mode is correct, continue with Startup Repair and the targeted boot-configuration checks above.
If the disk disappears from firmware or from WinRE, stop modifying BCD or boot sectors and obtain hardware diagnosis.
Use installation media safely when recovery is unavailable
A normal compatible USB flash drive is useful only as a way to start Windows installation media and reach WinRE. It does not change firmware mode, convert MBR to GPT, or repair a damaged ESP by itself.
Boot the USB in the same firmware mode you are troubleshooting, choose Repair your computer, and then open the recovery tools. If the USB itself appears twice in the boot list, choose the entry marked UEFI when repairing a UEFI installation. Keep the internal disk selected for repair and avoid the Windows Setup option that deletes or reformats partitions unless you have a verified backup and have deliberately chosen a clean installation.
Common mistakes that make recovery harder
- Switching modes repeatedly: First determine whether the disk is MBR or GPT. Repeated toggling obscures which configuration is known to work.
- Assuming every disk is Disk 0: Verify the target with DiskPart before running MBR2GPT.
- Formatting to “make UEFI work”: Formatting erases data and is unnecessary for an eligible MBR2GPT conversion.
- Deleting partitions to satisfy validation: MBR2GPT’s partition-count and layout checks are safety gates, not an invitation to remove partitions.
- Ignoring BitLocker: Suspend protection as required and keep the recovery key before conversion or WinRE repairs.
- Assuming Windows is always
C:in WinRE: Recovery assigns letters independently; identify the volume containing the Windows directory. - Using Bootrec as a universal fix: Match each command to the demonstrated failure and distinguish BIOS/MBR boot code from UEFI/ESP boot files.
- Repairing an invisible drive: If firmware or WinRE cannot see the OS disk, investigate connections, controller settings, and hardware first.
When to stop and contact the PC manufacturer
Contact the PC maker or a qualified technician when the internal drive is absent from firmware, remains absent from WinRE, reports repeated hardware errors, or requires manufacturer-specific storage-controller settings. Also stop if the machine cannot reliably enter firmware setup, the disk makes unusual noises, or the conversion utility reports a layout problem you cannot interpret safely.
Manufacturer-specific firmware labels, boot-entry behavior, Secure Boot defaults, and controller modes vary. Provide the technician with the exact firmware setting before the change, the current disk-detection result, the WinRE list disk output, and any MBR2GPT or Startup Repair error text. That information is more useful than repeatedly rerunning commands.
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