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:

* 128 partitions per disk: This is a vast improvement over MBR-based disks.
* 18 Exabyte (EB) volume size: This is a theoretical maximum because hard disk hardware is not yet available that supports such vast volume sizes.
* 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:

* 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.

* 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.

* 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.

* 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) 128MB 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.

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