GPT

Fixing problems of MBR

Advantages of GPT over MBR include

Logical Block Addresses (LBAs) are 64 bits rather than 32 bits (and is therefore not limited to 2 TiB = 2³²*512, 512 being the block size)
Supports many partitions (rather than just four primary partitions).
Provides both a primary and backup partition table for redundancy.
Uses version number and size fields for future expansion.
Uses CRC32 fields for improved data integrity.
Defines a GUID for uniquely identifying each partition (Hence the name GPT)
Uses a GUID and attributes to define partition content type.
Each partition contains a 36 character human readable name.
UEFI cannot boot from an MBR disk.

Layout on harddisk

A GPT disk starts with a 512 byte large protective MBR (where an ordinary MBR would be) to prevent MBR-only partitioning tools from overwriting GPT disks.

This protective MBR contains an entry to an inexisting 2 TiB large partition (with code EE00). Thus, the MBR-only partitioning tool thinks the entire disk is occupied.

When executing gdisk /dev/sda, it might print (note the Found valid GPT with protective MBR):

GPT fdisk (gdisk) version 1.0.0

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help):

The GPT header is stored in the 2nd sector (LBA 1) and is followed by the GPT partition entry array.

The minimum size reserved for the GPT partition entry array is 32 blocks (sectors?), i.e. with block size 512 = 16 KiB.

The first partition (usually) starts with sector 2048.

ESP

A GPT formatted harddisk must have one EFI system partition (ESP).

Partition type

With parted: parted /dev/disk set partition-number bios_grub on

With gdisk: use partition type 0xEF02.

Otherwise, set GUID to 21686148-6449-6e6f-744e656564454649.

Links

What's a GPT?.

GPT

Fixing problems of MBR

Layout on harddisk

ESP

Partition type

See also

Links