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A master boot record (MBR) is a legacy disk partitioning structure: it stores information about where partitions begin and how large they are, and it plays a role in the traditional BIOS startup process. It is not simply another name for a bootloader. On Windows systems, MBR is also contrasted with GPT, the partition style required for the drive containing Windows in Microsoft’s UEFI deployment guidance.
What the master boot record contains
The MBR is a record at the start of a disk that includes a partition table describing partition locations and lengths. The operating system uses that information to identify volumes, and the record is read during startup to help locate the operating system. Microsoft describes the role of basic disks and their partition information.
In the traditional BIOS-based Windows boot arrangement, the MBR partition table marks an active partition, which the startup process uses to continue booting. That boot role is one part of MBR; the term also names the disk’s legacy partition style. Microsoft’s BIOS/MBR partition guidance explains this arrangement.
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An MBR partition table has four entries. In Windows, those entries can describe four primary partitions, or three primary partitions and one extended partition. The extended partition can contain logical drives, so the four-entry limit does not mean an MBR disk can contain only four logical drives. Microsoft’s Windows and GPT FAQ documents this Windows partitioning model.
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MBR vs. GPT in Windows
GPT is the newer partition style used in Microsoft’s UEFI-based Windows deployment guidance. For Windows deployed to a UEFI device, Microsoft requires GPT on the drive that contains Windows; the guidance allows up to 128 partitions. Microsoft also describes GPT as able to manage disks larger than 2 TB, while its conversion overview characterizes MBR as limited to disks of 2 TB or smaller. These are Windows documentation limits and guidance, not claims about every possible implementation. See Microsoft’s UEFI/GPT partition guidance and its overview of converting between partition styles.
| Point of comparison | MBR | GPT in Microsoft Windows guidance |
|---|---|---|
| Typical boot context | Legacy BIOS-based Windows boot arrangements use an active partition. | Windows deployed to a UEFI device uses GPT for the drive containing Windows. |
| Partition layout | Four partition-table entries; Windows can use an extended partition to hold logical drives. | Up to 128 partitions. |
| Disk size guidance | Microsoft describes MBR as limited to disks of 2 TB or smaller. | Microsoft says GPT can manage disks larger than 2 TB. |
These differences do not mean every UEFI computer categorically cannot start from an MBR disk. Firmware and deployment configuration matter; the specific rule above is Microsoft’s guidance for Windows installed on a UEFI device.
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Should you convert an MBR disk?
Understanding what MBR means does not require changing a disk. If you are considering conversion, first confirm the computer’s firmware mode, the operating system’s requirements, and which disk contains Windows. Back up important data before making partition changes.
Windows documents distinct conversion workflows with different conditions. The Disk Management procedure described by Microsoft requires the disk to have no partitions or volumes. The MBR2GPT tool supports conversion without modifying or deleting data when its requirements are met; that does not make conversion risk-free or remove the need to check compatibility. Microsoft’s Disk Management conversion guidance and its MBR2GPT guidance cover these separate approaches.
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