Convert MBR to GPT while cloning to a new SSD

A PC that boots in BIOS mode from an MBR disk is getting a new SSD: this is the right time to migrate to GPT and UEFI. Here is how to do it without putting the customer’s data at risk.

Why migrate to GPT

Many PCs installed with Windows 7 and later upgraded to Windows 10 still boot in legacy BIOS mode (Legacy or CSM), from a disk partitioned as MBR. It works, but:

Windows 11 requires a UEFI boot on a Secure Boot capable PC: a PC on MBR will not pass the check;
an MBR disk cannot handle more than 2 TB: beyond that, the space cannot be used;
MBR only allows four primary partitions and a single partition table, with no backup copy.

Is the disk MBR or GPT?

Three ways to check on the customer’s PC:

Disk Management: right-click the disk, Properties, Volumes tab, “Partition style” line.
Diskpart, in a Command Prompt run as administrator: diskpart, then list disk. An asterisk in the “Gpt” column means GPT; no asterisk means MBR.
System Information (msinfo32): the “BIOS Mode” line says Legacy for a BIOS boot, and “UEFI” otherwise.

First: can the PC boot in UEFI mode?

A GPT disk only boots in UEFI mode. Most PCs sold since Windows 8 (2012) can, but they may have been set to Legacy mode at the factory or by a technician. Go into the BIOS and look for a “Boot Mode” setting (UEFI or Legacy), “CSM” or “Launch CSM”. If the BIOS only offers Legacy mode, stay on MBR: the clone would not boot.

Microsoft’s method: MBR2GPT

Windows 10 and 11 have a built-in tool, mbr2gpt.exe, that converts the disk in place, without erasing the data. You run it from the command prompt of a Windows installation USB drive (Shift+F10), or from Windows itself, as administrator, with the /allowFullOS option:

mbr2gpt /validate /disk:0 /allowFullOS
mbr2gpt /convert /disk:0 /allowFullOS

The first line only checks; the second converts. After that, the BIOS must be switched to UEFI mode.

The tool refuses the disk if it has more than three primary partitions, an extended partition, or a boot setup it does not recognise. This is often the case with branded PCs: the manufacturer adds its own partitions (on HP, “HP_TOOLS” and the factory restore partition; on Dell, the utility partition and the restore partition), and mbr2gpt /validate stops on a disk layout error. A BitLocker-encrypted disk must have its protection suspended first, and its protectors recreated afterwards. Above all, the tool modifies the customer’s only disk, and Microsoft warns that the conversion cannot be undone. If something goes wrong, there is no disk left that boots.

Keeping the same disk: the manual method

If the disk stays in the PC, here are the steps, in order. Warn the customer first: without the manufacturer’s partitions, the factory restore with the manufacturer’s key is gone.

Back up the customer’s data, or make an image of the disk.
Delete the manufacturer’s partitions, and only those: never the active boot partition (often “System Reserved”), nor the Windows partition. Disk Management often refuses to delete them; in diskpart: select disk 0, list partition, select partition N, then delete partition override. Check the number twice before you confirm.
If the freed space is right after C:, extend C: (Disk Management, right-click C:, Extend Volume). Otherwise, leave it free: it does not get in the way of the conversion.
Run the two mbr2gpt commands given above: /validate first, then /convert.
Switch the BIOS to UEFI, with CSM disabled, and boot.
Enable Secure Boot if the PC is moving to Windows 11.

Doing it while cloning

In the workshop, when you are replacing the disk anyway, there is a simpler and safer way: leave the old disk as it is, and create the new SSD directly as GPT. Nothing is written to the old disk. If the new one does not boot, you put the old one back and the customer leaves as before. And the manufacturer’s partitions no longer block anything: the new disk is built from scratch.

With MBI Clone

When the disk to clone is MBR, MBI Clone shows a “New disk boot format” choice at the first step:

Keep MBR / BIOS, selected by default: a standard clone, which will boot in Legacy mode like the old one;
Migrate to GPT / UEFI: only the new disk is created as GPT. The old one stays intact.

To migrate, you must tick a box: you confirm that the PC receiving the disk can boot in UEFI mode and that you will be able to disable Legacy mode on it. MBI Clone also checks on its side: a PC declared without UEFI blocks the migration, even with the box ticked. When it can read this, it also checks that Secure Boot will accept Windows at startup.

The new disk is built cleanly, never copied from the manufacturer’s layout: EFI partition, reserved partition, Windows, then the recovery partition at the end of the disk, so that C: can be extended later. The manufacturer’s partitions are not copied. If one of them holds a factory restore, MBI Clone says so before cloning: you back it up separately, or you agree to do without it, knowing what that means. At the end of cloning, MBI Clone reads back the partition table, the boot setup and the recovery environment before saying it has finished.

After cloning: set the BIOS

Fit the new SSD in place of the old one. Keep the old disk aside.
In the BIOS, switch booting to UEFI and disable CSM (or Legacy).
Boot from the new disk. Windows starts, with “Windows Boot Manager” in the boot list.
Once Windows has started, you can enable Secure Boot if the customer is moving to Windows 11.

Pitfalls

The BIOS was left in Legacy mode: the PC says “No bootable device” or goes back to the BIOS. Switch it to UEFI.
The disk comes from another PC: the workshop computer cannot tell whether the customer’s PC supports UEFI. Check in its BIOS before choosing GPT.
Extra partitions (data, manufacturer’s factory restore, extended partition): they are not carried over to the new disk. Copy their data separately before returning the PC. Without its restore partition, the PC can no longer go back to its factory state with the manufacturer’s key: a Windows installation USB drive takes its place.
BitLocker: at first boot, Windows may ask for the recovery key. Have it before you start.

Common questions

Does the customer lose any data?

No. With MBR2GPT, the data stays in place; during a clone, the old disk is not even modified.

Can you go back to MBR?

Not with MBR2GPT. After a clone, just put the old disk back and switch the BIOS back to Legacy.

Does Windows need to be reinstalled?

No. Windows is the same; only the boot changes.