UEFI vs Legacy Boot Mode Issue – How to Fix Windows Boot Errors
One of the most common reasons for Windows failing to boot is a UEFI and Legacy boot mode mismatch. If your PC suddenly shows errors like “Failed Boot Sequence”, “No Bootable Device”, or gets stuck at the manufacturer logo, the boot mode setting in BIOS/UEFI may no longer match how Windows was installed.
This guide explains what UEFI and Legacy modes are, why boot errors happen, and how to fix them safely without formatting.
What Is UEFI and Legacy Boot Mode?
UEFI (Unified Extensible Firmware Interface)
UEFI is the modern replacement for traditional BIOS. It supports faster boot times, Secure Boot, large disks (GPT), and is required for Windows 11.
- Uses GPT partition style
- Supports Secure Boot
- Recommended for Windows 10/11
Legacy (CSM / BIOS Mode)
Legacy mode is the older BIOS compatibility mode. It is mainly used by older operating systems or systems installed with MBR.
- Uses MBR partition style
- No Secure Boot support
- Common on older PCs
Why UEFI vs Legacy Causes Boot Errors
Windows can only boot if the boot mode matches the disk layout. When they don’t match, the system cannot find the Windows bootloader.
| Windows Installed As | BIOS Set To | Result |
|---|---|---|
| UEFI + GPT | Legacy / CSM | Boot failure |
| Legacy + MBR | UEFI | No Bootable Device |
This often happens after:
- BIOS/UEFI update or reset
- Changing Secure Boot settings
- Replacing SSD or cloning Windows
- Loading BIOS default settings
How to Check Your Current Boot Mode
Method 1: From BIOS/UEFI
- Restart the PC
- Enter BIOS (F2 / DEL / F10 depending on model)
- Look for Boot Mode, UEFI / Legacy, or CSM
Method 2: From Windows (If It Boots)
- Press Win + R
- Type msinfo32
- Check BIOS Mode (UEFI or Legacy)
Fix 1: Switch BIOS Boot Mode to Match Windows
If Windows was installed in UEFI mode, make sure BIOS is also set to UEFI. If Windows was installed in Legacy mode, BIOS must stay in Legacy / CSM.
Important: Do not randomly switch modes. Always confirm how Windows was originally installed.
Fix 2: Convert Legacy (MBR) to UEFI (GPT) Without Formatting
If you want to switch from Legacy to UEFI safely, Windows provides a built-in tool.
mbr2gpt /validate
mbr2gpt /convertAfter conversion:
- Enter BIOS
- Set Boot Mode to UEFI
- Disable Legacy / CSM
Fix 3: Repair Bootloader After Mode Change
If Windows still does not boot, use a Windows installation USB and run Startup Repair.
bootrec /fixmbr
bootrec /fixboot
bootrec /scanos
bootrec /rebuildbcdSecure Boot and UEFI Issues
Secure Boot can block startup if drivers or boot records are not trusted. If boot fails after enabling Secure Boot:
- Temporarily disable Secure Boot
- Confirm Windows boots correctly
- Update BIOS and drivers before re-enabling
FAQ
Should I use UEFI or Legacy?
For modern systems and Windows 10/11, UEFI is strongly recommended. Legacy should only be used for older systems.
Can switching boot mode delete my data?
Switching modes alone does not delete data, but an incorrect configuration can prevent booting. Always back up important files.
Why does Windows boot only in one mode?
Windows bootloader is created specifically for the mode used during installation.
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