Quick Guide: How to Use GPT fdisk to Convert MBR to GPT

Quick Guide: How to Use GPT fdisk to Convert MBR to GPT

Converting an MBR disk to GPT with GPT fdisk (gdisk) is a common task when preparing drives for modern OS installs or UEFI boot. This guide gives a concise, safe, step-by-step procedure for Linux, macOS, and Windows (gdisk available cross-platform). Always back up important data before changing partition tables.

Before you begin

• Backup: Copy any important files to another drive or cloud storage. Converting partition tables can risk data loss.
• Check OS/firmware: To boot from GPT, modern systems typically need UEFI. Older BIOS-only systems may require a BIOS-boot partition for GRUB.
• Install gdisk:
  • Linux: install via your package manager (e.g., apt install gdisk or yum install gdisk).
  • macOS: brew install gptfdisk
  • Windows: download gdisk (gdisk64.exe) from the project page and run as Administrator.

Quick checklist

• Identify target disk (device name, e.g., /dev/sda on Linux, .\PhysicalDrive0 on Windows).
• Note existing partitions and sizes.
• Decide whether you need an EFI System Partition (ESP) for UEFI boot (FAT32, ~100–512 MB) or a BIOS-boot partition for legacy BIOS+GRUB (~1–2 MB–1 GB as needed).
• Ensure you have recovery media (live USB) in case the system becomes unbootable.

Step-by-step conversion (preserve partitions where possible)

1. Identify the disk

   • Linux: sudo lsblk -f or sudo fdisk -l
   • macOS: diskutil list
   • Windows: diskpart -> list disk (or use Disk Management)

2. Launch gdisk on the target disk

   • Linux/macOS: sudo gdisk /dev/sdX
   • Windows: run gdisk64.exe and enter the disk number (e.g., 0) or use the device path.

3. let gdisk read the disk

   • gdisk will detect an MBR and report the protective GPT status. It may offer to convert MBR to GPT automatically.

4. Convert MBR to GPT

   • From gdisk main prompt:
     • If gdisk offers the automatic conversion, follow prompts.
     • Or use the recovery & transformation menu: press r then enter, then press f to “load MBR and build fresh GPT from it”. This imports existing MBR partitions into the GPT partition table.
   • Verify the partition layout with p (print table). Ensure partitions and start/end sectors match expectations.

5. Create or set an EFI System Partition if needed

   • If you need UEFI boot and there is no FAT32 ESP:
     • Use gdisk to create a new partition in free space (n), choose the size (e.g., +512M), then set its type code to EF00 (EFI System).
     • Alternatively, convert an existing small FAT32 partition to type EF00.
   • For BIOS-only systems using GRUB on GPT, create a small BIOS boot partition type EF02 (if required by your bootloader).

6. Write changes to disk

   • When the partition table looks correct, press w to write the new GPT to disk and exit. Confirm when prompted.
   • If you made a mistake or want to cancel, press q to quit without saving.

7. Post-conversion tasks

   • If you created an ESP, format and mount it:
     • Linux: mkfs.vfat -F32 /dev/sdXn and mount to /boot/efi for bootloader installation.
     • Windows: Windows will recognize an ESP when installing to GPT; otherwise, use Disk Management to format.
   • Reinstall or repair the bootloader if the system doesn’t boot:
     • Linux: reinstall GRUB (in UEFI mode use grub-install –target=x8664-efi –efi-directory=/boot/efi).
     • Windows: if converting a Windows system disk, use Windows repair tools (bootable installer -> Repair -> Command Prompt -> use bcdboot or diskpart + bootrec as needed). Note: converting system disks may require additional steps; many prefer Microsoft’s MBR2GPT tool for Windows system disks.
   • Reboot and select UEFI boot if applicable.

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sudo gdisk /dev/sda # at prompt:
p# print partition table
r             # recovery & transformation menu
f             # load MBR and build fresh GPT from it
p             # verify partitions
w             # write changes and exit