Deferences between Master Boot Record (MBR) and Globally Unique Identifier(GUID) Partition Table (GPT) Disk?

When you install a disk in a computer that is running Windows 7, you can choose to select one of two partitioning schemes 1. Master Boot Record (MBR)-based partitioning scheme 2. Globally unique identifier (GUID) partition table (GPT)-based partitioning scheme.

The following are common reasons to partition a disk:

1. Separate operating system files from data and user files
2. Place applications and data files in the same location
3. Put cache, log, and paging files in a location separate from other files
4. Create multiboot setup environments

You can use Disk Management to perform disk-related tasks such as creating and formatting partitions and volumes, and assigning drive letters. In addition, you can use the diskpart command, along with other command-line utilities, to perform disk management tasks such as partitioning disks or converting disks from one partition scheme to the other.

What is an MBR Disk ?

The Master Boot Record (MBR) contains the partition table for the disk and a small amount of executable code called the master boot code. A bootable hard disk that contains an MBR is an MBR disk. The MBR is created when the disk is partitioned, is on the first sector of the hard disk, and contains a four-partition entry table describing the size and location of a partition on a disk using 32-bit Logical Block Address (LBA) fields. The size of the partition cannot exceed 2 TB. Most Windows 7 platforms, such as 32-bit and 64-bit SKUs running on motherboards with Basic Input/Output System (BIOS) firmware, require an MBR partitioned system disk and are not bootable with a larger capacity disk.

How MBR-based disks work ?

The MBR is stored at a consistent location on a physical disk, enabling the computer BIOS to reference it. During the startup process, the computer examines the MBR to determine which partition on the installed disks is marked as active. The active partition contains the operating system startup files.

Note: You can install the rest of the operating system on another partition or disk. In Windows 7, the active partition must contain the boot sector, boot manager, and related files.

Features of MBR-based disks

The MBR partition scheme has been around for a long time and supports both current and early desktop operating systems, such as the MS-DOS and the Microsoft® Windows NT® Server 4.0 operating system. Consequently, the MBR partition scheme is widely supported. However, the MBR partition scheme imposes certain restrictions. These include:

1. Four partitions on each disk: MBR-based disks are limited to four partitions. All of these can be primary partitions, or one can be an extended partition with logical volumes inside. You can configure the extended partition to contain multiple volumes.

2. A 2 Terabyte (TB) maximum partition size: A partition cannot be larger than 2 TB.

3. No redundancy provided: The MBR is a single point of failure, and if corrupted or damaged, it can render the operating system non-bootable.

Question: What are three restrictions of an MBR partitioned disk? Have you encountered these limitations in your organization, and if so, what did you do to work around them?

Answer : The restrictions are that MBR partitioned disks are limited to four partitions, a 2 TB maximum partition size, and there is no data redundancy provided.

Globally unique identifier partition table

What is a GPT Disk ?

As operating systems evolve and hard disks grow larger, the inherent restrictions of an MBR partitioned disk limit the viability of this partitioning scheme as an option in many scenarios. Consequently, a new disk partitioning system has been developed: Globally unique identifier (GUID) partition table or GPT.

GPT contains an array of partition entries describing the start and end LBA of each partition on disk. Each GPT partition has a unique identification GUID and a partition content type. Also, each LBA described in the partition table is 64-bits in length. The GPT format is specified by the Unified Extensible Firmware Interface (UEFI), but is not exclusive to UEFI systems. Both 32-bit and 64-bit Windows operating systems support GPT for data disks on BIOS systems, but they cannot boot from them. The 64-bit Windows operating systems support GPT for boot disks on UEFI systems.

GPT disks features

GPT-based disks address the limitations of MBR-based disks. GPT disks support:

a) 128 partitions per disk: This is a vast improvement over MBR-based disks.
b) 18 Exabyte (EB) volume size: This is a theoretical maximum because hard disk hardware is not yet available that supports such vast volume sizes.
c) Redundancy: The GPT is duplicated and protected by Cyclic Redundancy Checks (CRC).

You can implement GPT-based disks on Windows Server® 2008, Windows Vista, and Windows 7. You cannot use the GPT partition style on removable disks.

GPT architecture

On a GPT partitioned disk, the following sectors are defined:

a) Sector 0 contains a legacy protective MBR. The protective MBR contains one primary partition covering the entire disk.

b) The protective MBR protects GPT disks from previously-released MBR disk tools such as Microsoft MS-DOS FDISK or Microsoft Windows NT Disk Administrator.

These tools view a GPT disk as having a single encompassing (possibly unrecognized) partition by interpreting the protected MBR, rather than mistaking the disk for one that is unpartitioned.

a) Legacy software that does not know about GPT interprets only the protected MBR when it accesses a GPT disk.

b) Sector 1 contains a partition table header. The partition table header contains the unique disk GUID, the number of partition entries (usually 128), and pointers to the partition table.

c) The partition table starts at sector 2. Each partition entry contains a unique partition GUID, the partition offset, length, type (also a GUID), attributes, and a 36 character name.

d) The following table describes the partitions.

Partition	Type	Size	Description
A	EFI System Partition (ESP)	100 MB	Contains the boot manager, the files that are required for booting an operating system, the platform tools that run before operating system boot, or the files that must be accessed before operating a system boot. The ESP must be first on the disk. The primary reason for this is that it is impossible to span volumes when the ESP is logically between what you are attempting to span.
B	Microsoft Reserved (MSR)	128 MB	Reserved for Windows components. This partition is hidden in Disk Management and does not receive a drive letter. Usage example: When you convert a basic GPT disk to dynamic, the system decreases the size of the MSR partition and uses that space to create the Logical Disk Manager (LDM) Metadata partition.
C	Operating System (OS)	Remaining disk	Contains the OS and is the size of the remaining disk.

Question : How does a GPT partitioned disk on a 64-bit Windows 7 operating system use an MBR?

Answer : On a GPT partitioned disk, Sector 0 contains a legacy protective MBR. The protective MBR contains one primary partition covering the entire disk. The protective MBR protects GPT disks from previously released MBR disk tools such as Microsoft MS-DOS FDISK or Microsoft Windows NT Disk Administrator. These tools view a GPT disk as having a single encompassing (possibly unrecognized) partition by interpreting the protected MBR, rather than mistaking the disk for one that is unpartitioned. Legacy software that does not know about GPT interprets only the protected MBR when it accesses a GPT disk.