Switch from legacy MBR disk to GPT disk with Windows 10

Article
03/23/2023

To facilitate upgrading from downlevel operating systems, such as Windows 7, or transitioning from BIOS Boot to UEFI Boot for the enhanced security features, Microsoft has provided the following information on switching from legacy MBR disk to GPT disk with Windows 10

The steps in the following sections will enable a more seamless upgrade to Windows 10 and enable the user the ability to leverage the new and improved security features of Windows 10. For purposes of the below steps, we will refer to the GUID Partition Table as GPT, and legacy Master Boot Record as legacy MBR boot disks.

These four configurations will be used:

Config # 1 - UEFI boot without a Compatibility Support Module (CSM) or the CSM is disabled in firmware. Requires a GPT Hard Disk Drive (HDD).
Config # 2 - UEFI boot with CSM enabled, booting from GPT HDD
Config # 3 - Legacy BIOS boot with CSM enabled, booting from legacy MBR HDD
Config # 4 - UEFI boot with CSM enabled, booting from legacy MBR HDD

Upgrade paths

Existing OS and Config	Target OS and Config	Results	Security options
Windows 7 x64 installed to system with UEFI firmware, CSM enabled on GPT HDD (Config # 2)	Windows 10 x64 installed to UEFI firmware CSM enabled on GPT HDD (Config # 2)	Will boot and run Windows 10 OS	OS is able to make use of MS security features supported in firmware once CSM is disabled
Windows 7 x64 installed to BIOS with active partition NTFS HDD (Config # 3)	Windows 10 x64 installed to BIOS with active partition NTFS HDD (Config # 3)	Will boot and run Windows 10 OS	Only able to leverage Bitlocker
Windows 7 x86 installed to BIOS with active partition NTFS HDD (Config # 3)	Windows 10 x86 installed to BIOS with active partition NTFS HDD = works (Config # 3)	Will boot and run Windows 10 OS	Only able to leverage Bitlocker
Windows 7 x86 installed to BIOS with active partition NTFS (Config # 3)	Windows 10 x64 installed to BIOS with active partition NTFS HDD (Config # 3)	Will need installation media and clean install.	Only able to leverage Bitlocker (other security features require UEFI boot)
Windows 7 x64 installed to BIOS with active partition NTFS (Config # 3)	Windows 10 x64 installed to UEFI firmware CSM disabled on GPT HDD (Config # 1)	Special instructions below	OS is able to make use of MS security features supported in firmware

Definition of terms

Compatibility support module (CSM) can typically be enabled or disabled in firmware. This module facilitates, but does not dictate booting to an active partition with legacy master boot record (MBR). Depending on BIOS/Firmware boot options, you may be able to enable the CSM and still select to boot to UEFI boot mode using GPT disk or legacy MBR boot mode. Having the CSM enabled and loaded into memory is required for Windows 7 to boot UEFI. UEFI boot does not need CSM to be enabled. With CSM disabled, boot does not use an active partition on the Hard Disk Drive(HDD), it does make use of an EFI System Partition (ESP) where it looks for a recognized file system such as FAT-FAT32 with boot files. Boot files can be defined in either a) NVRAM (boot000n) or b) Using UEFI specification defined fallback boot method looking for \EFI\Boot\Boot(arch).efi (for example: bootx64.efi) This boot method does not work on a legacy MBR configured NTFS boot disk.

Legacy MBR boot is not able to recognize GUID Partition Table (GPT) disks. It requires an active partition and supporting BIOS to facilitate access to disk. OLD and limited on HDD size and number of partitions. On UEFI firmware systems, it requires CSM enabled and loaded into memory to facilitate active partition booting.