How to Fix the "Selected Disk is Not a Fixed MBR Disk" Issue

Some users reported encountering an error message when they attempted to set the primary partition as active using DiskPart: "The selected disk is not a fixed MBR disk. The ACTIVE command can only be used on a fixed MBR disk." They were unsure why this message appeared and wondered if there was a solution to this issue.

If you're seeing this error, too, this article will provide the information you need. You'll learn what causes this error message and how to fix it.

"The Reason for Choosing a Disk Not Being a Fixed MBR Disk Issue"

There can be many reasons behind the “Selected disk is not a fixed MBR disk” issue. We have listed the two most common ones below.

• The partition is formatted as GPT.

The active partition command only works with BIOS/MBR systems. If your disk is formatted as GPT and you're booting in UEFI mode, the computer may not recognize the active partition command.

• Bootloader issues.

This error message might be due to an incorrect booting procedure of the computer. You can resolve this issue by repairing the Boot Manager.

If you're confused about MBR and GPT, refer to the article about MBR vs GPT for more information.

How to Solve the "The Selected Disk is Not a Fixed MBR Disk" Issue

Now that you know the cause of the error message, you can resolve the issue by using one of the following solutions.

Method 1: Convert GPT to MBR

Since the boot command only works with BIOS/MBR systems, converting the partition from GPT to MBR might resolve the issue. You can perform the conversion using the DiskPart command prompt and Disk Management on your computer. However, both methods require deleting the volumes, which entails data loss.

Thus, we recommend using a third-party partition manager – AOMEI Partition Assistant Professional to solve the “The selected disk is not a fixed MBR disk” issue without data loss. This tool allows users to resize partitions, convert MBR disk to GPT or vice versa, and delete partitions for free in Windows, etc.

Please download and run the software on your computer, then follow these steps to convert GPT to MBR.

• Click "Disk Converter" from the left menu. Choose "Convert GPT to MBR" and then hit "Next" to proceed. • Select the GPT disk you want to convert and click "Convert". • Wait for the conversion process to finish.

Method 2: Repair the Boot Manager

You'll need your Windows installation drive for this. If you have the installer on a USB or DVD, proceed with the following steps to repair the Boot Manager.

Step 1. Boot the computer from the installation drive.

Step 2: On the Welcome screen, choose "Repair your computer."

Click on Repair your computer:

Step 3: Go to Troubleshoot > Command Prompt.

Step 4. Enter the following command:

• bootrec
• bootrec FixMbr
• bootrec FixBoot
• bootrec ScanOs
• RebuildBcd

Type each command and then press “Enter.”

Step 5. Wait for the installation to complete, and then remove the installation drive from your computer.

Step 6. Type exit and press Enter.

Now restart your computer and check if it can start up normally and if the “Selected disk is not a fixed MBR disk” error still exists.

Bottom line

The "selected disk is not a fixed MBR disk" issue usually occurs due to the conflict between the MBR and GPT partitions on your computer. Once you understand the differences between MBR and GPT, you'll know how to fix this problem. Partition Master is the best tool for managing partitions. With this tool, you can clone, format, and merge partitions without losing data.