Convert an MBR Disk to GPT Without Losing Windows

The conversion only makes sense for an **MBR system disk** with a legacy loader. Both facts come from one command each, before anything is changed.

Get-Disk | Select-Object Number,FriendlyName,PartitionStyle,IsBoot,IsSystem
bcdedit /enum {current} | Select-String -Pattern 'path|device'

**validate** is read-only and lists every reason the conversion would fail: extended or logical partitions, more than three primary partitions, not enough free space at the end of the disk.

mbr2gpt /validate /disk:0 /allowFullOS
Get-Partition -DiskNumber 0 | Select-Object PartitionNumber,Type,Size,Offset

The GPT header needs room at both ends of the disk. A partition that reaches the last sector blocks the conversion until you shrink it.

Get-Partition -DiskNumber 0 | Sort-Object Offset -Descending | Select-Object -First 1 PartitionNumber,Size,Offset
Get-Disk 0 | Select-Object Number,Size,LargestFreeExtent

The conversion rewrites the partition table; a power loss in that second is not recoverable by software. With the image in hand, run the convert command from a full Windows session.

wbadmin start backup -backupTarget:E: -include:C: -allCritical -quiet
mbr2gpt /convert /disk:0 /allowFullOS

The disk is GPT now, but the machine still starts in legacy mode until you turn off CSM. That single step is what turns the conversion into a UEFI boot.

:: firmware setup > Boot > CSM/Legacy > Disabled > save and exit
Get-Disk | Select-Object Number,PartitionStyle
bcdedit /enum {current} | Select-String -Pattern 'path'

A UEFI boot with a signed loader is the configuration that can use Secure Boot. Confirm both, then turn the firmware feature on.

Confirm-SecureBootUEFI
Get-Partition -DiskNumber 0 | Select-Object PartitionNumber,Type,Size
msinfo32