Operating systems typically access partitions through logical drives, each of which is assigned a logical drive letter. A primary partition contains only one logical drive, but an extended partition may be subdivided into one or more logical drives. An operating system only assigns logical drive letters to those partitions with a recognizable format, that is, supported by the operating system.
The assignment of drive letters is done by the operating system's volume manager when the operating system is loaded. The first physical disk drive is searched for a recognizable primary partition. This partition is assigned the logical drive letter "C" and the next physical disk is searched. After logical drives in primary partitions have been assigned for all physical drives, drive letters are assigned to the recognizable logical drives in extended partitions.
A different operating system may reside on each of the logical drives in an extended partition (with certain limitations; see Operating System Restrictions). Usually, DOS 5.0 and OS/2 V1.3 will only be installed on a primary partition on the first physical disk. An exception could be with DOS, for example, where there are no recognizable primary partitions on the first physical disk. In this case DOS (V4.1 or later) may recognize the primary partition(s) on the first physical drive to be formatted with the HPFS or Boot Manager, and treat them as unusable to DOS. In these situations, the first drive letter C: may be assigned to a primary partition on the next physical disk.
Logical drives in extended partitions are shareable; any data installed on these logical drives can be used by an operating system running from any other logical drive in the system, provided the partition formats are compatible. Although 26 logical drive letters are available (A: through Z:), A: and B: are typically reserved for diskette media. Under DOS or OS/2, up to 24 logical drives may be created using the logical drive support for extended partitions.