Need to convert my Windows 10 install from Legacy to UEFI

Hello Andrew Wilson, 

Thank you for reaching out to the Microsoft Q&A Forum! 

It sounds like you're using dual Windows 10 installations (SSD and HDD) both using Legacy BIOS boot mode and you need to convert the SSD installation to UEFI without affecting file access to the HDD installation. I'm here to help you resolve this. 

To convert your SSD-based Windows 10 installation from Legacy to UEFI, you need to convert the disk from MBR to GPT, create an EFI System Partition, update the bootloader for UEFI, and configure your motherboard’s firmware. Since WinRE is inaccessible, we’ll use alternative methods to perform the conversion without relying on WinRE. Below are the steps to achieve this while preserving your SSD install and ensuring access to HDD files. 

Method 1: MBR2GPT Conversion Tool (Recommended) 

1. Boot into SSD Windows 10 installation
2. Open Command Prompt (Admin) in Windows
3. Identify SSD disk number: diskpart > list disk
4. Validate conversion possibility: (Replace X with your SSD disk number)

mbr2gpt /validate /disk:X /allowFullOS

5. If validation passes, convert: (Replace X with your SSD disk number)

mbr2gpt /convert /disk:X /allowFullOS

6. Reboot and change BIOS to UEFI mode
7. Disable CSM/Legacy Support in BIOS 

Method 2: Clean Installation Approach (If MBR2GPT fails): 

1. Backup important data from SSD
2. Create Windows 10 installation media (USB/DVD)
3. Boot from installation media in UEFI mode
4. Delete all partitions on SSD during installation
5. Install Windows 10 fresh (will auto-create GPT)
6. Restore data from backup 

Method 3: Manual Partition Conversion (Advanced users only): 

1. Boot from Windows 10 installation media 
2. Press Shift+F10 to open Command Prompt 
3. Use diskpart to manually convert: 

• diskpart 
• list disk 
• select disk X (your SSD) 
• Clean 
• convert gpt 
• create partition efi size=100 
• format quick fs=fat32 
• assign letter=S 
• create partition primary 
• format quick fs=ntfs 
• assign letter=C 

4. Install Windows to C: partition 

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Warm regards,    

Hendrix Vu | Microsoft Q&A Support Specialist 

Dear Andrew Wilson,

Thank you for the detailed background on your dual-boot setup and your goal to convert your Windows 10 SSD installation from Legacy BIOS to UEFI. You've done a great job navigating a complex configuration, and you're right — most guides assume a simpler, single-install environment.

Summary of Your Current Setup

Two Windows 10 installations: one on SSD (your main OS) and one on HDD (legacy, minimal use).

Both are installed in Legacy BIOS mode.

You want to convert the SSD install to UEFI, and you're not concerned about the HDD install.

WinRE is currently unavailable or non-functional on the SSD install.

Boot menu shows three entries, but only the SSD and HDD entries successfully boot.

 Recommended Steps to Convert SSD Install to UEFI

To convert your SSD-based Windows 10 installation to UEFI, follow these steps carefully:

1. Verify GPT Partition Style

UEFI requires the disk to use GPT, not MBR.

Open Disk Management (diskmgmt.msc)

Right-click your SSD > Properties > Volumes tab

Check the Partition Style: If it's MBR, you'll need to convert it to GPT.

If it's already GPT, skip to Step 3.

2. Convert MBR to GPT (if needed)

Use the built-in mbr2gpt tool:

Open Command Prompt as Administrator

Run: cmd mbr2gpt /validate /disk:<disk number> /allowFullOS

If validation passes, run: cmd mbr2gpt /convert /disk:<disk number> /allowFullOS

Replace <disk number> with the number of your SSD (check in Disk Management).

This will convert the disk to GPT and update boot files.

3. Enable UEFI in BIOS

After conversion:

Reboot and enter BIOS/UEFI settings

Change boot mode from Legacy/CSM to UEFI

Disable Legacy Boot if needed

Ensure the SSD is set as the primary boot device

4. Repair Bootloader (if needed)

If Windows doesn’t boot after switching to UEFI:

Create a Windows 10 installation USB

Boot from it in UEFI mode

Choose Repair your computer > Troubleshoot > Command Prompt

Run: cmd bootrec /fixboot

bootrec /scanos

bootrec /rebuildbcd

If WinRE is unavailable on your SSD install, this external USB method is the best workaround.

Additional Notes

The “Windows 10” entry that doesn’t boot may be a leftover or broken bootloader entry. You can clean this up later using bcdedit or tools like EasyBCD.

Once your SSD install is running in UEFI mode, you can safely ignore or remove the HDD install from the boot menu.

Let me know if you'd like help creating a bootable USB or cleaning up the boot entries afterward. Happy to walk you through it!

Best regards, 

Domic Vo

There are third party software that can convert MBR2GPT directly or indirectly.

They typically have a fee and can be used for other purposes.

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It may be possible to convert using diskpart.

And it may be possible to create a working Windows RE.

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First post images of disk management and share files for analysis.

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1)Disk management

https://www.tenforums.com/tutorials/2503-how-post-screenshot-disk-management.html

(widen rows and columns > make sure that text is not cut off and all text can be viewed)

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2)Run the V2 log collector > post a share link

https://www.tenforums.com/bsod-crashes-debugging/2198-bsod-posting-instructions.html

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3)Run: diskparinfo > post a share link

https://www.tenforums.com/attachments/bsod-crashes-debugging/355861d1640707960-batch-files-use-bsod-debugging-diskparinfo.bat

https://www.tenforums.com/bsod-crashes-debugging/162488-batch-files-use-bsod-debugging.html#post1994162

https://www.tenforums.com/bsod-crashes-debugging/162488-batch-files-use-bsod-debugging.html#Part3

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2)Run: Tuneup plus > post a share link

https://www.tenforums.com/attachments/bsod-crashes-debugging/360137d1645183388-batch-files-use-bsod-debugging-tuneup_plus_log.bat

https://www.tenforums.com/bsod-crashes-debugging/162488-batch-files-use-bsod-debugging-2.html#post2177966

https://www.tenforums.com/bsod-crashes-debugging/162488-batch-files-use-bsod-debugging.html#Part3

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3)Run: mbr2gpt > post a share link

https://www.tenforums.com/attachments/bsod-crashes-debugging/360509d1645510785-post-problem-reports-here-batch-files-use-bsod-debugging-info_on_mbr2gpt.bat

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Clean install can be performed anytime as long as you're willing to backup any important files, reinstall drivers and applications. No troubleshooting is usually needed except for one driver. In contrast files can be troubleshooted for a possible MBR2GPT.