The Selected Disk Is Not a Fixed MBR Disk: What It Means and How to Fix It

The diskpart error "The selected disk is not a fixed MBR disk. The ACTIVE command can only be used on fixed MBR disks" appears when you run active on a partition that is on a GPT disk or a removable drive. The active command only works on MBR disks.

Part 1. Why This Error Appears

GPT disks use UEFI booting — there is no "active partition" concept. Instead, the EFI System Partition (ESP) handles boot. The active command is a legacy BIOS/MBR concept.

💡 Tip: To check if your disk uses MBR or GPT: in diskpart, run list disk. If the disk has an asterisk (*) in the "GPT" column, it uses GPT partitioning. If no asterisk, it uses MBR.

Part 2. Fix Windows Boot on a GPT Disk

For GPT disks, use bcdboot instead of marking a partition active:

1. Boot from a Windows USB → Repair your computer → Command Prompt
2. Identify your Windows partition letter (usually C:)
3. Find the EFI partition and assign it a letter:

diskpart
list vol
select vol X   (select the EFI partition — FAT32, ~100-500 MB)
assign letter=Z
exit

4. Rebuild boot files:

bcdboot C:\Windows /s Z: /f UEFI

💡 Tip: If you're unsure which volume is the EFI partition, look for a FAT32 partition between 100 MB and 500 MB in size — that's the EFI System Partition. Larger FAT32 partitions are usually recovery or USB volumes, not the ESP.

Part 3. If You Actually Need MBR (Legacy Systems)

If your older system requires MBR and you want to convert from GPT:

⚠️ Important: Converting from GPT to MBR using convert mbr in diskpart destroys all partition data on the disk. Back up all data first. This is rarely the right solution for modern systems.

For true legacy BIOS systems, the conversion process:

1. Back up all data
2. diskpart → select disk X → clean → convert mbr
3. Recreate partitions and reinstall Windows

Part 4. Common Context: Trying to Make USB Bootable

If you got this error while trying to make a USB drive bootable:

• USB drives are removable — the active command doesn't apply
• Use Rufus (rufus.ie) to create bootable USBs — it handles partition configuration automatically
• Or use diskpart: clean → create partition primary → format fs=fat32 quick → active only works if you first convert to MBR: convert mbr

💡 Tip: When creating a bootable USB with Rufus, select the correct partition scheme: GPT for UEFI-only systems (most modern PCs after 2015), MBR for older Legacy BIOS systems. Choosing the wrong scheme creates a USB that some systems cannot boot from.

🗣️ r/techsupport user: "Got this error trying to mark a partition active on my main drive for Windows boot repair. Turns out my drive is GPT not MBR. Used bcdboot instead and fixed the boot issue without converting to MBR."

🗣️ r/sysadmin tip: "On any modern UEFI system (2012+) with GPT disks, forget about the 'active' command — it doesn't apply. bcdboot is the correct tool for rebuilding UEFI boot entries on GPT disks."

FAQ

What does "the selected disk is not a fixed MBR disk" mean? The active command in diskpart only works on MBR-partitioned disks. Your disk uses GPT partitioning (or is a removable drive), so the command doesn't apply. GPT UEFI systems don't use the "active partition" concept.

How do I make a partition bootable on a GPT disk? Use bcdboot from Windows Recovery Command Prompt: bcdboot C:\Windows /s Z: /f UEFI (where Z: is the EFI system partition). This rebuilds boot entries on GPT/UEFI systems.

Can I convert my GPT disk to MBR? Yes — but only by erasing all data on the disk first. Use diskpart → clean → convert mbr. Only do this if you specifically need MBR for an older BIOS system.

Why does diskpart show an asterisk (*) next to my disk? In diskpart's list disk output, an asterisk in the "GPT" column indicates the disk uses GPT partitioning. No asterisk means MBR.

References

• Microsoft — bcdboot Command
• Microsoft — diskpart
• Rufus — Bootable USB Creator
• r/techsupport — GPT Boot Repair
• r/sysadmin — bcdboot vs active