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Stands for "Master Boot Record." An MBR is a small section of a hard disk or other storage device that contains information about the disk. It is located in the boot sector and defines the disk partitions as well as the code used to start the boot sequence.

There are several variations of MBRs, but they are all 512 bytes in size and contain a partition table and bootstrap code. Modern master boot records may contain disk signatures, timestamps, and other disk formatting information. Additionally, while early MBRs only supported four partitions, newer versions can support up to sixteen. However, all master boot records are limited to 512 bytes, which means they can only address up to two terabytes of data. Therefore, disks that are formatted using an MBR are limited to 2TB of usable disk space.

The master boot record was first used by PC-DOS compatible computers in 1983 and was the standard way to format DOS disks for over two decades. However, most hard drives are now using the GUID partition table (GPT), which is compatible with multiple operating systems and does not have a 2TB limit.

NOTE: If necessary, a master boot record can be added to a GUID partition table for backwards compatibility.

Updated: August 30, 2012

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