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.
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.
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.
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.