How to Fix The Selected Disk is not a Fixed MBR Disk?

The error message The Selected Disk is not a Fixed MBR Disk indicates the commands you’re trying to use are suitable only for MBR disks, and you’re dealing with a GPT disk. The “Active” command you are trying to use is not applicable to a GPT disk.

This error usually appears in several circumstances. It often shows up when trying to set a primary partition to active on a GPT disk—a concept not recognized by GPT (GUID Partition Table).

Additionally, if the boot manager or BCD (Boot Configuration Data) store has problems, this error message is likely to show up.

Understanding the error message requires knowledge of UEFI, BIOS, MBR, and GPT.

UEFI (Unified Extensible Firmware Interface) and BIOS (Basic Input/Output System) connect hardware to the operating system on motherboards. They serve a similar purpose but with distinct differences.

MBR (Master Boot Record) and GPT (GUID Partition Table) represent two different types of partitioning on Windows PCs. To know more about them, visit MBR VS GPT.

MBR is compatible with Legacy BIOS, while GPT is for UEFI. Attempting to run MBR-only commands on GPT disks, such as setting a GPT disk to Active, will result in the error message.

With the causes explained, let’s explore solutions.

1. Fix the BCD store

The computer’s firmware uses the BCD store at startup to locate Windows for booting up. If the BCD store is corrupted or removed, you might encounter error messages like this one.

BCD commands can help fix a damaged BCD store by copying boot files from the Windows system drive and remaking the lost BCD store, which may resolve the issue.

1. Insert a Windows bootable USB drive. Make one if needed using this guide here.
2. Go to boot options and choose the USB drive. Press the Delete key to open boot options.
3. Boot from USB, choose your language, and click Next.
   
4. Select Repair your PC.
   
5. Choose Troubleshoot then Command Prompt.
   
6. The BCD store is recreated with this command. Take care when entering it, as the system drive isn’t always C:, and system partition can vary.

   bcdboot<source>/I <locale>/s <volume-letter>/f <firmware type>
   Example: bcdboot C:/Windows/I en-us/s S:/f All

7. Determine the system drive and system partition with these commands.

   diskpart
   list disk

8. Select your OS disk by size and input:

   Select disk 0

   Note: Note that 0 is my OS disk, insert the number specific to your system that you see in the command prompt.
   

9. List volumes by typing list volume. Note down these volumes, as you’ll identify the one with Windows installed.
10. Exit diskpart with exit.
11. Enter dir <volume letter>:\ to find the system drive.
    
12. Note the system drive letter for later. Now, find a volume letter for the system partition. Go back to diskpart, select the OS disk, and type list volume. Then, select the fat32 volume.
    
13. Assign a letter to the fat32 volume with assign letter=z.
    
14. Exit diskpart with exit.
15. Now, ready to use the BCD command.
16. Replace <source> with your OS drive letter and <volume-letter> with what you assigned.

    bcdboot<source>/I <locale>/s <volume-letter>/f <firmware type>

17. For example, I’ll use bcdboot D:/Windows/I en-us/s Z:/f All
    
18. The error should now be corrected.

2. Convert the GPT disk to MBR

You can solve the error by converting a GPT disk to MBR, but this will erase data. Make sure you back up important files first before using this approach.

For non-OS drives, use the command prompt on your PC or the Windows Recovery Environment steps 1–5 from the first method for OS drives.

Converting your GPT OS disk to MBR will require Windows re-installation and enabling bios legacy mode for the MBR disk.

1. Type these commands.

   diskpart
   list disk
   select disk <disk number causing the error> (e.g., select disk 0)
   convert MBR

   

2. Now, MBR commands will work without error messages.

3. Fix the Boot Manager

If the Boot Manager of your system is causing issues, it might result in the error message. The Boot Manager is an essential component that helps your computer start or boot correctly.

This method requires you to input specific commands to rebuild the Boot Configuration Data (BCD), fix the Master Boot Record (MBR), and repair the boot function, helping to make sure a smoother, error-free boot sequence.

1. Open the command prompt using steps 1–5 from the first method.
2. Go to the system disk with C:.
3. Enter cd Windows\system32.
4. Input the commands to repair the bootloader.

   bootrec /RebuildBcd
   bootrec /fixMbr
   bootrec /fixboot

   

5. Check if the problem is solved.

If these methods haven’t worked, you can switch the bios mode from UEFI to legacy following this guide. You’ll need to convert your GPT OS disk to MBR first using the second method and then you can reinstall Windows on the MBR disk, resolving the error.