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Primary / extended / logical partition

The partition table in the partition sector of the MBR can contain at most four entries. This could be either only primary partitions (max. four) or exactly one extended partition and up to three primary partitions. This convention allows, for example, that on a second hard disk only one extended partition is created.

Primary partition:
A primary partition is completely defined by its entry in the partition table. This entry contains the information about the beginning and end of the partition, the type of the partition and the position of the accompanying boot sector.
Primary partition

Extended partition:
In the case of an extended partition the entry in the partition table of the MBR describes only the space reserved for the extended partition. This space can be split in up to 24 logical partitions. Logical partitions can be created only inside an extended partition. In simple words, the extended partition represents the space reserved for the logical partitions. The partition entry in the MBR points to the partition sector of the first logical partition, which resides directly at the beginning of the extended partition.
Extended partition

Logical partition

A logical partition -- often also called a logical drive -- is a set of adjoining sectors inside an extended partition. In many respects the logical partitions resemble primary partitions. However the boot loader in the standard MBR of Microsoft does not support booting from logical partitions. With the help of a boot manager a logical partitions can be easily booted, supposed the operating system allows itself to be installed on a logical partitions.

The size, position and type of logical partitions is stored in a chain of partition sectors inside the extended partition. The partition table of a logical partition sector contains two entries. One entry describes the logical partition, the other entry contains the position of the partition sector of the next logical partition. The entry in the partition table of the MBR describes just the beginning of this chain. If the chain is broken, the following logical partitions will not be accessible any more.
Logical partition