MBR disks are a type of disk boot sector that contain information about how the disk partitions are organized. MBRs also contain information about the operating system boot procedure. MBR is an old system, introduced in March 1983, and it originally supported FAT12 on DOS 2.0. Later, with the introduction of DOS 3.0, it was upgraded to support FAT16 filesystems as well. It has several limitations, one of which is that the maximum addressable storage space is 2 TB.

An MBR disk supports a maximum of four primary partitions. Alternatively, you can create up to three primary partitions, an extended partition, and up to 128 logical drives within the extended partition.

On an MBR disk, the partition and boot data is stored in an entry saved in one place only. If this data is corrupted or erased, even partially, you could lose all the data stored on a disk.

The following screenshot shows Disk 1, which is initialized as an MBR disk. It has three primary partitions (assigned drive letters E, F, and G) and one logical drive (drive letter H) within an extended partition. Disk 2 is initialized as a GPT disk, with five primary partitions: