Master Boot Record

aliases:

A hard disk partition style that holds information about how the disk has it's partitions setup and the file system that each partition is using. It's only able to address storage spaces less than 2 TB in size.

In MBR, partitions are identified by 16-bit entries that make up the partition table located in the master boot record (crazy, eh?) of that drive. While Windows assigns letters for every partition, Linux just mounts the partition into a directory that is accessible by the end user.

MBR limits the amount of partitions that can be created on a single hard disk drive to four partitions. These partitions are called primary partitions.

Extended Partitions

An exception to the 4 partition limit that MBR imposes is manifested in the spirit of the fabled extended partition. However, there are special rules that they follow:

cannot be bootable
OS files can't be stored in an extended partition
limit ONE per storage device
cannot be added to a storage device that already has the maximum of four primary partitions
cannot be directly formatted; must create logical partitions within the extended partition

Logical Partitions

Residents of extended partitions. You can create up to 24 logical partitions on an extended partition, essentially breaking the 4 partition limit. Once created, the logical partitions can be formatted as if they were a primary partition with the file system. The OS will not know the difference between a logically extended partition and a primary partition.

Active Partitions

The main hard drive of any computer system needs at least one active partition. The active partition is the partition where the OS's files are stored, which are first loaded by the system bootloader. If the OS files are in a partition that isn't marked as active, the system will fail to boot to that OS.

Boot Process

BIOS loads the master boot record into memory
MBR reads the partition table to find the primary partition that's marked as active
MBR uses partitioning info from the table to find sector zero of the active primary partition, which is the boot sector
From there, MBR loads the boot application for the OS stored in the boot sector. If loaded successfully, BIOS turns over control of the rest of the boot process to the OS

Troubleshooting

Since the MBR is responsible for locating the active partition and loading everything from there, any damage to the MBR can produce one or multiple of the following symptoms:

"Corrupt MBR" errors.
"Invalid partition table" errors
"Error loading OS" errors
"Missing OS" errors