GPT and MBR partition styles

Windows setup refuses your disk, and the culprit is a table you never see: the partition style. Three letters decide whether your PC starts at all.

The short answer

GPT and MBR are the two partition styles, the ways a disk records where its partitions are. MBR or GPT – which one do you need? MBR, the Master Boot Record from the BIOS era, holds four partition records with 32-bit sector fields [1][2]. GPT, the GUID Partition Table, comes from the UEFI specification, uses 64-bit addresses and keeps a backup of its table [1][2]. Windows boots in UEFI mode only from GPT disks, and Windows 11 requires UEFI firmware [2][5]. Our assessment: use GPT for every new disk and keep MBR only for old hardware that cannot start in UEFI mode.

How it works

1. MBR. The legacy MBR sits in the first sector and holds four partition records; each stores the start sector and the size as a 32-bit value [1]. Microsoft notes that MBR disks support only four partition table entries, and more need an extended partition [2]. Our calculation: 2^32 sectors of 512 bytes are 2 TiB, the most an MBR entry can describe on a disk with 512-byte sectors.
2. GPT. GPT uses 64-bit logical block addresses [1]. The primary GPT header sits in block 1 and a backup header in the last block of the disk, and CRC32 checksums protect the header and the partition entry array [1]. The specification reserves at least 16,384 bytes for that array, with 128 bytes per entry, and Windows allows 128 partitions per GPT disk [1][2].
3. Protective MBR. A GPT disk still carries an MBR in its first sector. This protective MBR contains a single partition of type 0xEE that spans the disk, so that old tools which don't understand GPT see the disk as occupied instead of empty [1][2].
4. Boot modes. To boot a PC in UEFI mode, Windows needs a GPT drive, while legacy BIOS mode requires MBR [3]. Windows does not support booting UEFI from MBR disks [2], and Microsoft generally recommends installing in UEFI mode because it includes more security features than legacy BIOS mode [6].
5. Converting. MBR2GPT converts a disk from MBR to GPT without modifying or deleting data. It requires at most three primary partitions, no extended or logical partitions and free space for the GPT structures at the start and end of the disk; if needed, it creates a new EFI system partition [4]. After the conversion, the firmware must be switched to UEFI mode [4].
6. Windows 11. Windows 11 requires UEFI firmware that is Secure Boot capable [5]. An older PC whose Windows still starts from MBR in legacy mode therefore needs GPT and UEFI before the upgrade.

A practical example

Your Windows 10 PC starts in legacy mode from an MBR disk, and you want Windows 11. In PowerShell as administrator, Get-Disk lists your disks with their number and partition style [4]. Then let MBR2GPT check disk 0 without changing anything; /allowFullOS permits the run inside normal Windows instead of Windows PE [4]:

mbr2gpt /validate /disk:0 /allowFullOS

Only if validation passes: back up the disk, suspend BitLocker, run the same command with /convert instead of /validate, and switch the firmware to UEFI at the next start [4]. This is a generic walk-through, not a report from our own PC.

Checklist

• Check the partition style of your system disk before you change any firmware setting.
• Back up the disk before you convert it.
• Run mbr2gpt with /validate first and use /convert only when validation passes [4].
• Suspend BitLocker before the conversion and recreate its protectors afterwards [4].
• Switch the firmware to UEFI right after the conversion [4].
• Initialize new disks as GPT; MBR only makes sense for legacy BIOS systems [3].