[Solved!]The Selected Disk Is Not A Fixed MBR Disk on Windows 11/10

Some Windows users fail to mark a partition as active in Disk Management or in Diskpart utility and receive the message saying that "the selected disk is not a fixed MBR disk." Therefore, you need to solve this error so that you can continue to mark the desired partition as active and use it as a bootable partition.  

What does the selected disk is not a fixed MBR disk mean?

When you cannot mark partition as active windows 10 and it prompts 'the selected disk is not a fixed MBR disk.' To understand the error in depth, you need to know what are UEFI, BIOS, MBR, and GPT in the first place.

BIOS(Basic Input/Output System) and UEFI(Unified Extensible Firmware Interface) are two different types of motherboard firmware, in charge of connecting the computer's hardware to its operating system during the booting process. 

MBR(Master Boot Record) and GPT(GUID Partition Table) are two different types of tables maintained on disks by the OS and are used to outline and describe the partitions on that disk. MBR is associated with BIOS boot mode, and GPT is associated with UEFI boot mode.

If you attempt to mark a GPT partition as active on Windows 11/10, you will receive the notification, given that the UEFI method can't recognize concepts such as an active partition, and the active command can only be used on fixed MBR disks. A fixed MBR disk is a drive inside your computer with MBR format.

In a nutshell, 'the selected disk is not a fixed MBR disk' means the disk you selected is a GPT disk that cannot be marked as an active partition. 

What causes the selected disk is not a fixed MBR disk?

In case you have formatted the partition as GPT, then you could see the error 'the selected disk is not a fixed MBR disk' in the Diskpart utility when you mark it as active. In addition, you may have enabled UEFI boot mode on your computer, and the problems with the boot manager may also be the culprit of the not a fix MBR disk error.

How to fix the selected disk is not a fixed MBR disk?

After knowing the meaning and the causes of 'the selected disk is not a fixed MBR disk,' here we can proceed to the solutions to this error. Then, you can mark the partition you prefer as active and boot your Windows computer from it. Just apply the following solutions one by one to get rid of the issue.

Disable UEFI in BIOS

If the boot mode is set to UEFI instead of the legacy BIOS, disabling UEFI may work to get this issue solved. Here's how:

Restart your Windows computer and enter into BIOS by pressing the BIOS key(F1, F2, Del, etc) when it is about to start.

After rebooting the computer, you can check whether this issue has been fixed. This solution may also help when Windows says the operating system is not found when booting up.

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Fix the boot manager

The boot manager on Windows is a piece of software that loads the operating system when your boot up the Windows computer. Some users have successfully fixed the selected disk is not a fixed MBR disk by fixing the damaged boot manager. 

Convert GPT to MBR

If you have formatted the disk as GPT, you cannot mark the partition located on a GPT disk as active, because GPT-associated UEFI boot mode cannot recognize the concept of an active command. Thus, you need to convert GPT to MBR to fix the error message 'the selected disk is not a fixed MBR disk.'

The conversion from GPT to MBR automatically deletes all contents on the disk. Therefore, you should back up your disk in advance to avoid data loss. To guarantee the success of the conversion, you'd better run the command of cleaning disk to remove data from the disk. Let's dive into the details.