Windows and GPT FAQ

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.

MBR disks support only four partition table entries. For more than four partitions, a secondary structure known as an extended partition is necessary. Extended partitions can then be subdivided into one or more logical disks.

Windows creates MBR disk partitions and logical drives on cylinder boundaries based on the reported geometry, although this information no longer has any relationship to the physical characteristics of the hardware (disk driver or RAID controller). Starting with Windows Vista and Windows Server 2008, more logical boundaries are selected when the hardware provides better hints at the true cache or physical alignment. Because this partition information is stored on the drive itself, the operating system is not dependent on the alignment.

MBR partitioning rules are complex and poorly specified. For example, does cylinder alignment mean that each partition must be at least one cylinder in length? An MBR partition is identified by a two-byte field, and coordination is necessary to avoid collision. IBM originally provided that coordination, but today there is no single authoritative list of partition identifiers.

Another common practice is using partitioned or "hidden" sectors to hold specific information by using undocumented processes and results in problems that are difficult to debug. In the past, vendor-specific implementations and tools were released to the public, which made support difficult.

GPT disks allow for growth. The number of partitions on a GPT disk isn't 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.

Chapter 5 of the Unified Extensible Firmware Interface (UEFI) specification (version 2.3) defines the GPT format. This specification is available at https://www.uefi.org/specifications.

Basic disks are the most commonly used storage types with Windows. "Basic disk" refers to a disk that contains partitions, such as primary partitions and logical drives, usually formatted with a file system to become a volume for file storage.

The protective MBR area on a GPT partition table exists 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.

For more information about basic disks, see Basic and Dynamic Disks.

Dynamic disks were first introduced with Windows 2000 and provide features that basic disks don't, such as the ability to create volumes that span multiple disks (spanned and striped volumes) and the ability to create fault-tolerant volumes (mirrored and RAID-5 volumes). Dynamic disks can use the MBR or GPT partition styles on systems that support both. For more information about dynamic disks, see Basic and Dynamic Disks.

GPT disks are self-identifying. All the information needed to interpret the partitioning scheme of a GPT disk is completely contained in structures in specified locations on the physical media.

In theory, a GPT disk can be up to 2^64 logical blocks in length. Logical blocks are commonly 512 bytes in size.

The maximum partition (and disk) size depends on the operating system version. Windows XP and the original release of Windows Server 2003 have a limit of 2TB per physical disk, including all partitions. For Windows Server 2003 SP1, Windows XP x64 edition, and later versions, the maximum raw partition of 18 exabytes can be supported. (Windows file systems currently are limited to 256 terabytes each.)

The specification allows an almost unlimited number of partitions. However, the Windows implementation restricts this to 128 partitions. The number of partitions is limited by the amount of space reserved for partition entries in the GPT.

No. However, all GPT disks contain a Protective MBR.

The Protective MBR, beginning in sector 0, precedes the GPT partition table on the disk. The MBR contains one type 0xEE partition that spans the 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 are not aware of GPT and don't know how to properly access a GPT disk. Legacy software that does not know about GPT interprets only the Protected MBR when it accesses a GPT disk. These tools will 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.

This occurs when you use an MBR-only-aware disk tool to access the GPT disk. For more information, see the following sections:

• How a disk can be both GPT and MBR
• Protective MBR
• Why the GPT has a Protective MBR

Windows disk support

Windows XP x64 Edition can use GPT disks for data only.

No. The 32-bit version will see only the Protective MBR. The EE partition will not be mounted or otherwise exposed to application software.

Starting with Windows Server 2003 Service Pack 1, all versions of Windows Server can use GPT partitioned disks for data. Booting is only supported for 64-bit editions on Itanium-based systems.

Yes, all versions can use GPT partitioned disks for data. Booting is only supported for 64-bit editions on UEFI-based systems.

No. Again, legacy software will see only the Protective MBR.

You can move, or migrate, data-only GPT disks to other systems that are running Windows XP (64-bit edition only) or later versions of the operating system (32- or 64-bit editions). You can migrate data-only GPT disks after the system has been shutdown or after the safe removal of the disk.

GPT and MBR disks can be mixed on systems that support GPT, as described earlier. However, you must be aware of the following restrictions:

• Systems that support UEFI require that boot partition must reside on a GPT disk. Other hard disks can be either MBR or GPT.
• Both MBR and GPT disks can be present in a single dynamic disk group. Volume sets can span both MBR and GPT disks.

Removable media must be MBR, GPT, or "superfloppy."

Removable media without either GPT or MBR formatting is considered a "superfloppy". The entire media is treated as a single partition.

The media manufacturer performs any MBR partitioning of removable media. If the media has an MBR, only one partition is supported. There is little user-discernible difference between MBR-partitioned media and superfloppies.

Examples of removable media include floppy disk drives, JAZ disk cartridges, magneto-optical media, DVD-ROM, and CD-ROM. Hard disk drives on external buses such as SCSI or IEEE 1394 are not considered removable.

What is the default behavior of Windows XP 64-Bit Edition Version 2003 when partitioning media?

For Windows XP 64-Bit Edition Version 2003 only (for Itanium-based systems), fixed disks are partitioned by using GPT partitioning. GPT disks can be converted to MBR disks only if all existing partitioning is first deleted, with associated loss of data.

Only MBR disks can be used.

There is no inherent mapping between drive letter and partition that can be used to determine one from the other. A basic data partition must be identified by its partition GUID.

ESP partitions can be created by using the UEFI firmware utility Diskpart.efi or the Windows command line utility Diskpart.exe.

You shouldn't directly change any partition header entry. Don't use disk tools or utilities to make alterations or changes.

Detachable disks are typically expected to migrate between computers or simply to be unavailable to the operating system at times. Examples of detachable disks are USB disks, which can be easily disconnected by the end-user. Windows XP supports only MBR partitioning on detachable disks. Later versions of Windows support GPT partitions on detachable disks.

For more about removable media, see the following questions:

• Removable media
• Superfloppy

Windows GPT required partitions: EFI System Partition

The ESP contains the NTLDR, HAL, Boot.txt, and other files that are needed to boot the system, such as drivers. The Partition GUID defines the ESP:

DEFINE_GUID (PARTITION_SYSTEM_GUID, 0xC12A7328L, 0xF81F, 0x11D2, 0xBA, 0x4B, 0x00, 0xA0, 0xC9, 0x3E, 0xC9, 0x3B)

No, MBR disks can also have ESPs. UEFI specifies booting from either GPT or MBR. The ESP on an MBR disk is identified by partition type 0xEF. However, Windows does not support booting UEFI from MBR disks or 0xEF partitions.

The ESP is approximately 100MBs.

Such a configuration shouldn't be created, and is not supported in Windows.

ESP partitions can be replicated for high-availability configurations. Replication must be done manually and the contents must be synchronized manually when using software volumes. Hardware vendors may provide additional solutions for high availability. ESP partitions cannot be mirrored.

Microsoft places the HAL, loader, and other files that are needed to boot the operating system in the ESP.

The ESP should be first on the disk. The primary benefit to placing the ESP first, is that it is impossible to span volumes when the ESP is logically between the two data partitions that you are attempting to span.

The ESP should only include files that are required for booting an operating system, platform tools that run before operating system boot, or files that must be accessed before operating system boot. For example, files that are required for performing pre-boot system maintenance must be placed in the ESP.

Other value-add files or diagnostics used while the operating system is running should not be placed in the ESP. It is important to note that the space in the ESP is a limited system resource; its primary purpose is to provide storage for the files that are needed to boot the operating system.

The preferred option is for system manufacturers to place value-add contents in an OEM-specific partition. Just like MBR OEM partitions, the contents of GPT OEM (or other unrecognized) partitions are not exposed (given drive letters or returned in volume lists). Users are warned that deleting the partition can cause the system to fail to operate. An OEM-specific partition should be placed before the MSR and after any ESP on the disk. Although not architectural, this placement has the same benefits as placing the ESP first. For example, it is also impossible to span volumes when an OEM-specific partition is logically between the two data partitions that you are attempting to span.

Placement in the ESP is an option for applications or files that execute in the pre-operating system boot environment. However, the ESP is architecturally shared space and represents a limited resource. Consuming space in the ESP should be considered carefully. Files that are not relevant to the pre-operating system boot environment should not be placed in the ESP.

The Microsoft Reserved Partition (MSR) reserves space on each disk drive for subsequent use by operating system software. GPT disks do not allow hidden sectors. Software components that formerly used hidden sectors now allocate portions of the MSR for component-specific partitions. For example, converting a basic disk to a dynamic disk causes the MSR on that disk to be reduced in size and a newly created partition holds the dynamic disk database. The MSR has the Partition GUID:

DEFINE_GUID (PARTITION_MSFT_RESERVED_GUID, 0xE3C9E316L, 0x0B5C, 0x4DB8, 0x81, 0x7D, 0xF9, 0x2D, 0xF0, 0x02, 0x15, 0xAE)

Every GPT disk must contain an MSR. The order of partitions on the disk should be ESP (if any), OEM (if any) and MSR followed by primary data partition(s). It is particularly important that the MSR be created before other primary data partitions.

The MSR must be created when disk-partitioning information is first written to the drive. If the manufacturer partitions the disk, the manufacturer must create the MSR at the same time. If Windows partitions the disk during setup, Windows creates the MSR.

After the disk is partitioned, there will be no free space left to create an MSR.

When initially created, the size of the MSR depends on the size of the disk drive:

• On drives less than 16GB in size, the MSR is 32MB.
• On drives greater than or equal to 16GB, the MSR is 128 MB.

As the MSR is divided into other partitions, it becomes smaller.

Windows GPT ESP implementation

For UEFI systems, the boot drive must contain an ESP, an MSR, and at least one basic data partition that contains the operating system. Only one ESP should exist on a system even if multiple operating systems are installed on that system. In a mirrored boot configuration there may actually be two drives with an ESP but they are considered to be a redundant copy of the same ESP. Each data drive must contain at least an MSR and one basic data partition.

All basic data partitions on the drive should be contiguous. As noted above, placing an OEM-specific or other unrecognized partition between data partitions imposes limitations on later volume spanning.

Basic data partitions correspond to primary MBR partitions 0x6 (FAT), 0x7 (NTFS), or 0xB (FAT32). Each basic partition can be mounted using a drive letter or mount point, other volume device object, or both. Each basic data partition is represented in Windows as a volume device object, and optionally as a mount point or a drive letter.

It has the following partition type GUID

DEFINE_GUID (PARTITION_BASIC_DATA_GUID, 0xEBD0A0A2L, 0xB9E5, 0x4433, 0x87, 0xC0, 0x68, 0xB6, 0xB7, 0x26, 0x99, 0xC7);

The ESP partition isn't hidden, but also doesn't have an assigned drive letter. It will not appear in Explorer unless a drive letter gets assigned to it, but some tools will be able to list it.

Users will not see these partitions exposed in Windows Explorer, nor is any recognized file system exposed to legacy programs such as Context Indexing. The OEM-specific and other unrecognized partitions will be visible only in the Disk Management MMC snap-in since they will not have a recognizable file system.

Windows exposes only basic data partitions. Other partitions with FAT file systems may be mounted, but not exposed only programmatically. Only basic data partitions are assigned drive letters or mount points.

The ESP FAT file system is mounted, but not exposed. This allows programs running under Windows to update the contents of the ESP. Assigning a drive letter to the ESP using mountvol /s will allow access to the partition. Access to the ESP requires admin privilege. Although the MSR, and any partitions created from the MSR, could have recognizable file systems, none are exposed.

Any OEM-specific partitions or partitions associated with other operating systems are not recognized by Windows. Unrecognized partitions with recognizable file systems are treated like the ESP. They will be mounted, but not exposed. Unlike MBR disks, there is no practical difference between OEM-specific partitions and other operating system partitions; all are "unrecognized."

The user can use disk management tools such as the Disk Management utility or the diskpart.exe Windows command line. The MSR and any partitions created from the MSR are only visible from the command line.

Dynamic disks use two different GPT partitions

• A data container partition that corresponds to the MBR partition 0x42, with the following GUID: DEFINE_GUID (PARTITION_LDM_DATA_GUID, 0xAF9B60A0L, 0x1431, 0x4F62, 0xBC, 0x68, 0x33, 0x11, 0x71, 0x4A, 0x69, 0xAD);

• A partition to contain the dynamic configuration database, with the following GUID: DEFINE_GUID(PARTITION_LDM_METADATA_GUID, 0x5808C8AAL, 0x7E8F, 0x42E0, 0x85, 0xD2, 0xE1, 0xE9, 0x04, 0x34, 0xCF, 0xB3);

Volumes are created in the data container and mounted by default. Again, this is exactly the same as the contents of 0x42 MBR partitions.

For a drive to be eligible for conversion to dynamic, all basic data partitions on the drive must be contiguous. If other unrecognized partitions separate basic data partitions, the disk can't be converted. This is one of the reasons that the MSR must be created before any basic data partitions. The first step in conversion is to separate a portion of the MSR to create the configuration database partition. All non-bootable basic partitions are then combined into a single data container partition. Boot partitions are retained as separate data container partitions. This is analogous to conversion of primary partitions.

Windows XP and later versions of Windows differ from Windows 2000 in that basic and extended partitions are preferentially converted to a single 0x42 partition, rather than being retained as multiple distinct 0x42 partitions as on Windows 2000.

Yes. For more information, see What about mixing and matching GPT and MBR disks on the same system?

You can access the GPT disk partitions of different types using the tools that are listed in the following table.

By using the Microsoft Platform SDK APIs, you can also develop your own tools to access the GPT disk partitions at their primitive levels.

GPT and MBR disks are managed the same way. Disks can be formatted as GPT or MBR by using the Diskpart.exe command prompt utility or by using the Disk Administrator snap-in. Volumes can be created on both GPT and MBR disks, and both kinds of disks can be mixed in the same dynamic disk group.

Starting with Windows XP, there is no FTdisk set support on Windows for MBR or GPT disks. The only support for logical volumes is through dynamic disks.

Microsoft offers MBR2GPT.exe which converts disks from MBR to GPT.

NTFS is recommended on all basic data partitions and all dynamic volumes. Windows Setup and the Disk Management snap-in offer only NTFS. To circumvent that, the partition or volume must be formatted explicitly via the Format command-line tool.

Manipulating GPT disks and their contents

You can create a GPT disk only on an empty, unpartitioned disk (raw disk or empty MBR disk). For more information about creating GPT disks, see Using GPT Drives.

You can convert an existing partition format to another format. For more information, see the following TechNet articles:

• Change a Master Boot Record Disk into a GUID Partition Table Disk
• Change a GUID Partition Table Disk into a Master Boot Record Disk

The Disk and Partition GUIDs will no longer be unique. This must never happen. You can make a sector-by-sector copy of the contents of ESP or basic data partitions.

You can copy a whole GPT disk using the OPK imaging tools; however, there are some key caveats. The OEM Preinstallation Kit (OPK) initializes the Disk and Partition GUIDs to zero. On first boot of Windows, the operating system generates unique GUIDs. The OPK only supports generation of ESP, MSR, and basic data partitions.

If an application has recorded any Disk or Partition GUIDs it may break. Any applications, drivers, utilities, or firmware implementations supplied by system manufacturers or application vendors that rely on GUIDs should be capable of handling GUIDs that change from the OPK initialization values to those generated by the operating system.

The Diskpart.efi MAKE command is a way for OEMs to simplify operating system preinstallation and system recovery. This command can easily be extended to create a "default" disk configuration for the platform. For example, the system manufacturer could extend the MAKE command to automatically partition the boot drive with an ESP, MSR, an OEM-specific partition, and one basic data partition.

For example, consider a possible disk configuration called BOOT_DISK. In the event of business failure recovery, MAKE BOOT_DISK would allow the customer to completely repartition a boot disk to the original factory defaults.

If Windows detects a duplicate Disk or Partition GUID, Windows will generate new GUIDs for any duplicate Disk GUID, MSR Partition GUID, or MSR basic data GUID upon detection. This is similar to the duplicate MBR signature handling in Windows 2000. Duplicate GUIDs on a dynamic container or database partition cause unpredictable results.