MBR and GPT! What are they?!

Introduction

Mechanical hard drives have increased in size tremendously
over the past few years. Today, you can buy drives that are as big as 2TB, 3TB,
even 4TB in internal as well as external forms. 5TB drives are expected to hit
the market sometimes this year as well.

So, apart from additional storage, which we all love, what
else is there to know about these drives?

Basically, the only thing one should worry about with these
large drive is the method of formatting. You see, a standard method of
formatting the drive is typically called the MBR. A drive formatted as an MBR
uses Master Boot Record. This is a small sector at the beginning of every drive
that defines how that drive behaves, what partitions it looks for, etc. The
major limitation of MBR is that it cannot see partitions beyond 2TB or 2000GB.

Enter GPT. GPT is similar to MBR, but it's a bit more
extensive and does not have the same limitations. In other words, if you want
to use a hard drive with more than 2TB of space and create single partition,
you want to format that drive as GPT.

Here's some background information on the MBR.

The MBR, the most
important data structure on the disk, is created when the disk is partitioned.
The MBR contains a small amount of executable code called the master boot code,
the disk signature, and the partition table for the disk. At the end of the MBR
is a 2-byte structure called a signature word or end of sector marker, which is
always set to 0x55AA. A signature word also marks the end of an extended boot
record (EBR) and the boot sector.

The disk
signature, a unique number at offset 0x01B8, identifies the disk to the
operating system. Windows 2000 uses the disk signature as an index to store and
retrieve information about the disk in the registry subkey:

HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices

Master Boot
Code

The master boot
code performs the following activities:

1. Scans the partition table for the
   active partition.


2. Finds the starting sector of the
   active partition.


3. Loads a copy of the boot sector from
   the active partition into memory.


4. Transfers control to the executable
   code in the boot sector.

If the master
boot code cannot complete these functions, the system displays one of the
following error messages:

• Invalid partition table


• Error loading operating system


• Missing operating system

As for GPT, here's some background information on that:

The GUID
Partition Table (GPT) was introduced as part of the Unified Extensible Firmware
Interface (UEFI) initiative. GPT provides a more flexible mechanism for
partitioning disks than the older Master Boot Record (MBR) partitioning scheme
that was common to PCs.

A partition is a
contiguous space of storage on a physical or logical disk that functions as if
it were a physically separate disk. Partitions are visible to the system
firmware and the installed operating systems. Access to a partition is
controlled by the system firmware before the system boots the operating system,
and then by the operating system after it is started.

GPT disks can
grow to a very large size. The number of partitions on a GPT disk is not
constrained by temporary schemes such as container partitions as defined by the
MBR Extended Boot Record (EBR). The GPT disk partition format is well defined
and fully self-identifying. Data critical to platform operation is located in
partitions and not in unpartitioned or "hidden" sectors. GPT disks
use primary and backup partition tables for redundancy and CRC32 fields for
improved partition data structure integrity. The GPT partition format uses version
number and size fields for future expansion.

Each GPT
partition has a unique identification GUID and a partition content type, so no
coordination is necessary to prevent partition identifier collision. Each GPT
partition has a 36-character Unicode name. This means that any software can
present a human-readable name for the partition without any additional
understanding of the partition.

Basic disks are
the storage types most often used with Windows. The term basic disk refers to a
disk that contains partitions, such as primary partitions and logical drives,
and these in turn are usually formatted with a file system to become a volume
for file storage.

The protective
MBR area exists on a GPT partition table for backward compatibility with disk
management utilities that operate on MBR. The GPT header defines the range of
logical block addresses that are usable by partition entries. The GPT header
also defines its location on the disk, its GUID, and a 32-bit cyclic redundancy
check (CRC32) checksum that is used to verify the integrity of the GPT header.
Each entry in the GUID partition table begins with a partition type GUID. The
16-byte partition type GUID, which is similar to a System ID in the partition
table of an MBR disk, identifies the type of data that the partition contains
and identifies how the partition is used, for example, whether it is a basic
disk or a dynamic disk. Note that each GUID partition entry has a backup copy.