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Logical Drives and Partition TablesThe basic unit of a FAT file system is a logical drive. A logical drive is a contiguous portion (or possibly all) of a disk consisting of the following components in the given order:
Diskettes, and all devices formatted as diskettes, will contain exactly these components. With the introduction of MS-DOS 2.0, hard disks with much larger capacity than diskettes needed to be supported. Since MS-DOS was not able to handle such large volumes with a single logical drive, the concept of a partition table was added. The partition table only appears on hard disks. The MBR partition table can contain up to 4 entries, each entry describing a single logical drive in a portion of the disk. A special partition type, the Extended Partition, can implement a linked list of any number of logical drives. Each logical drive described by a partition has exactly the format described above: boot record, FAT(s), root directory, and data area. For backward compatibility, the first sector of a hard disk still has the basic structure of a boot record; it just adds the MBR partition table to the end of the sector. such a boot record with a partition table is called a master boot Record (MBR). If a hard disk is booted, the master boot record is executed which in turn will scan the partition table for a bootable logical drive. If one is found, the boot record of that logical drive is loaded and executed. The MBR partition table can hold only 4 partition records, and each is limited to 32 bit sectors. These limitations where overcome by the GUID Partition Table (GPT). It uses 64 bit sector numbers and can hold up to 128 partitions.
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