Convert MBR to GPT and create/repair EFI

Scope

Use this procedure when:

• the Windows system disk is currently MBR
• the machine is booted from Sergei Strelec WinPE
• the target is to boot Windows in UEFI mode
• Paragon will be used for MBR to GPT conversion
• diskpart and bcdboot will be used for EFI work
• data is present on the disk

Warning

Backup required: This procedure is written for a disk that contains data and is intended to preserve it. Partition and boot changes still carry risk. Proceed only after verifying a good backup.

Important notes

• This procedure is written for a disk that contains data and must be changed with minimum impact.
• Do not use diskpart convert gpt on a disk with data.
• Confirm the machine supports UEFI boot.
• Confirm you are working on the correct disk.
• In WinPE, Windows may not be on C:.
• Do not delete existing partitions unless absolutely necessary.
• This procedure always creates a new EFI partition after shrinking the Windows volume.
• The MSR partition is optional and does not affect boot success.

Variables

Placeholder	Meaning
DISKNO	Windows physical disk number
OSVOL	Windows volume number
S:	EFI partition drive letter
W:	Windows drive letter in WinPE

Step 1 - Identify the correct disk and Windows partition

Open Command Prompt and run:

diskpart
list disk
select disk DISKNO
list partition
list volume
exit

Find the real Windows drive letter:

dir C:\Windows
dir D:\Windows
dir E:\Windows
dir F:\Windows

Record:

• DISKNO
• OSVOL
• W:

Step 2 - Convert the disk from MBR to GPT with Paragon

1. Open Paragon from Sergei Strelec
2. Select the correct Windows disk
3. Run Convert MBR disk to GPT disk
4. Apply the operation
5. Close Paragon

Verification / if unsure

Run:

diskpart
list disk
exit

The disk should show a * under the GPT column.

Step 3 - Shrink the Windows volume

Shrink the Windows volume to create space for the new EFI partition.

Run:

diskpart
select volume OSVOL
shrink desired=300 minimum=260
exit

Notes:

• This procedure assumes you will always shrink the Windows volume before creating the EFI partition.
• desired=300 creates enough room for a 260 MB EFI partition and usually leaves a small margin.
• If shrink fails, verify the correct volume is selected and try again carefully.

Verification / if unsure

Run:

diskpart
select disk DISKNO
list partition
exit

Confirm that unallocated space now exists on the disk.

Step 4 - Create the EFI partition

Create a new EFI system partition in the space created by the shrink.

Run:

diskpart
select disk DISKNO
create partition efi size=260
format quick fs=fat32 label="EFI"
assign letter=S
exit

Notes:

• The EFI partition must be FAT32.
• It is used by UEFI firmware to load Windows boot files.
• This procedure creates a new EFI partition and then writes fresh boot files to it.

Verification / if unsure

Run:

diskpart
list volume
select disk DISKNO
list partition
exit

Confirm:

• S: is assigned to the EFI partition
• the EFI partition is FAT32
• the EFI partition is about 260 MB

Step 5 - Optional: create the MSR partition

The MSR partition is the Microsoft Reserved Partition.

What it is:

• a small reserved GPT partition used internally by Windows
• it is not formatted
• it does not get a drive letter
• it is not required for booting

When to create it:

• create it if you want the disk layout closer to a standard Microsoft GPT layout
• create it if your internal standards require it
• create it only if you have enough free space and want the extra layout completeness

When to skip it:

• skip it if the main goal is safe conversion with minimum changes
• skip it if space is limited
• skip it if you only need the machine to boot properly in UEFI mode

Run only if you want to create it:

diskpart
select disk DISKNO
create partition msr size=16
exit

Verification / if unsure

Run:

diskpart
select disk DISKNO
list partition
exit

Confirm that a small 16 MB MSR partition exists.

Step 6 - Write UEFI boot files

Run:

bcdboot W:\Windows /s S: /f UEFI

Expected result:

Boot files successfully created.

Verification / if unsure

Check the Windows path:

dir W:\Windows

Check that EFI boot files exist:

dir S:\EFI
dir S:\EFI\Microsoft
dir S:\EFI\Microsoft\Boot

If bcdboot fails:

• confirm W:\Windows is the correct Windows path
• confirm S: is the FAT32 EFI partition
• run bcdboot again after correcting the drive letters

Step 7 - Reboot in UEFI mode

1. Enter BIOS/UEFI setup
2. Change boot mode from Legacy/CSM to UEFI
3. Make sure Windows Boot Manager is available
4. Put Windows Boot Manager first if needed
5. Remove the USB stick and boot from the internal disk

Verification / if unsure

If the machine does not boot:

• confirm the disk is GPT
• confirm BIOS/UEFI is set to UEFI
• confirm Windows Boot Manager is selected
• if needed, boot back to WinPE and run:

bcdboot W:\Windows /s S: /f UEFI

Completion checklist

• Paragon converted the disk to GPT
• Windows volume was shrunk successfully
• EFI partition exists and is FAT32
• bcdboot completed successfully
• Firmware is set to UEFI
• Windows boots without the USB stick

Printable Field Sheet

MBR to GPT Conversion and EFI Repair/Create - Field Checklist

Backup required. Use only after verifying a good backup.

Step	Check	Done
1	Booted from Sergei Strelec WinPE	[ ]
2	Correct disk and Windows partition identified	[ ]
3	Disk converted with Paragon	[ ]
4	Windows volume shrunk successfully	[ ]
5	New EFI partition created and assigned letter S:	[ ]
6	MSR partition created if required	[ ]
7	bcdboot completed successfully	[ ]
8	BIOS/UEFI changed to UEFI and Windows Boot Manager selected	[ ]
9	System boots without USB	[ ]

Step 1 - Boot from USB Boot from Sergei Strelec WinPE.

Step 2 - Identify disk and Windows letter Find the correct disk and Windows partition.

diskpart
list disk
select disk DISKNO
list partition
list volume
exit

dir C:\Windows
dir D:\Windows
dir E:\Windows
dir F:\Windows

Step 3 - Convert disk with Paragon Convert the correct disk from MBR to GPT.

Step 4 - Shrink Windows volume Create space for the new EFI partition.

diskpart
select volume OSVOL
shrink desired=300 minimum=260
exit

Step 5 - Create new EFI partition Create the new UEFI boot partition.

diskpart
select disk DISKNO
create partition efi size=260
format quick fs=fat32 label="EFI"
assign letter=S
exit

Step 6 - Optional MSR partition MSR = Microsoft Reserved Partition. Not needed for boot. Create only if your standard requires it or you want a more complete GPT layout. Skip it if the goal is minimum change and reliable boot repair.

diskpart
select disk DISKNO
create partition msr size=16
exit

Step 7 - Write UEFI boot files Write Windows boot files to the EFI partition.

bcdboot W:\Windows /s S: /f UEFI

Step 8 - Set firmware boot mode Set firmware to UEFI. Select Windows Boot Manager.

Step 9 - Final boot test Remove the USB stick and confirm Windows boots from the internal disk.