Over time, storage requirements have grown from a few megabytes (MB) to millions of terabytes (TB). In turn, hard-disk storage sizes have also increased in proportion with storage requirements. With 1TB hard disks now common and 3TB disks soon available, system administrators now face problems with the legacy MBR partitioning scheme. It is now important to learn how your Windows Server Operating System supports the GPT scheme so you can be fully prepared to move to a new partitioning scheme, the GUID Partition Table (GPT).

Understanding the Limitations of MBR scheme:
The Master Boot Record (MBR) partitioning scheme is legacy partitioning scheme that is used by most operating systems. The MBR itself resides entirely on the first sector (512 bytes) of a hard disk. Of that space, the first 440 bytes of the MBR are devoted to the boot loader. The BIOS reads this code and executes it when the computer boots. This code area is followed by data space used to store information about the four partitions, known as Primary partitions. Each partition is described in two ways: using cylinder/head/sector (CHS) notation and using logical block addressing (LBA) notation. The CHS notation is obsolete because of its 24-bit limit which can only address 8GB of disk space. The 32-bit LBA values permits us to address up to 2TB. Because we don’t have any space left in the MBR to increase LBA size; this caps MBR access at only 2TB of disk space.

The other disadvantage of MBR is that the total data storage space of a computer hard disk can be divided into at most four partitions. If we need to have more than four partitions on a hard disk then we need to create a special type of partition called an extended partition. These partitions have data-integrity problems since they are a single data structure that's vulnerable to damage caused by carelessness or hardware failure. Additionally the extended partition data structure doesn’t have any form of error-detection capability, so damage can be difficult to spot.

GUID Partition Table scheme, a New Standard:
The GUID Partition Table (GPT) is a new standard for the layout of the partition table on a physical hard disk. It is a part of the Extensible Firmware Interface (EFI) standard proposed by Intel as a replacement for the PC BIOS. GPT uses the 64 bit disk pointers, which allow for a maximum disk partition size of 9.4 Zeta bytes, or 9.4 billion Tera Bytes.

GPT Guid Partition Table diagram
Another benefit of using GPT based disks includes overcoming the 4 primary partition limit of a MBR disk. GPT supports a maximum of 128 primary partitions. GPT data structures are also well defined and stored twice on the disk: once at the start and again at the end. This improves the odds of successful data recovery resulting from damage caused by an accident or a bad sector. Also, cyclic redundancy check (CRC) values are computed for critical data structures, improving the odds of detecting of data corruption. The figure below shows the layout of a GPT disk and how the data structure is stored.

Not all Windows OSes provide full support for GPT disks. From Windows Server 2003 SP1 onwards, GPT disks can be used as data disks but not as the boot disk. From Windows Server 2008 onwards, only x64 versions of the Microsoft OS support GPT disks as the boot disk in EFI (Extensible Firmware Interface) boot mode. GPT disks are not supported as the boot disk in BIOS Mode when booting to Windows OSes.

The table below provides information about the basic level of support provided by Microsoft OSes for each of these partition schemes.

MBR Disk GPT disk
Support in legacy OSes like DOS, Windows 98 etc. Yes No
Support for more than 2TB (Tera Byte) No Yes
Support as Data disk in x86 version of OS Yes Yes
Support as Data disk in x64 version of OS Yes Yes
Support as Boot disk in x86 version of OS Yes No
Support as Boot disk in x64 version of OS Yes Yes
Support for more than 4 primary partition No Yes (Supports upto 128 Partition)
Booting support through BIOS mode Yes No
Booting support through uEFI mode No Yes

Choosing Between Partition Schemes

The main considerations for choosing between the partition schemes are hard drive size, operating system version and software tools used in the operating system.
For users of older Microsoft Operating Systems running on hard drive sizes smaller than 2 TB, that use legacy software tools (which only understand the MBR scheme), the MBR partition scheme is your best option. For users of newer operating systems such as Windows Server 2008 R2, Windows Server 2008, or Windows Small Business Server 2008 that run their OSes on hard drives larger than 2 TB, the GPT partition scheme can be used.

It is recommended that if you can, you should use the GPT partition scheme beacuse it is more flexible, supports larger hard drive sizes, and has a number of features and recovery mechanisms that are not available with the MBR partition scheme.

About GPT disk - http://www.microsoft.com/whdc/device/storage/gpt_faq.mspx
Overview of Disk Management - http://technet.microsoft.com/en-us/library/dd163558.aspx
Partitions and Volumes - http://technet.microsoft.com/en-us/library/dd163559.aspx