PC Hardware


Partitions are logical divisions of a hard drive. A computer might have only one physical hard drive (called hard drive 0), but it can have anywhere from one to 24 logical drives, identified as C to Z.

Partitions exist for two reasons:

  • To divide the disk into several drive letters to make it easier to organize data files. Some users separate data, programs, and operating-system files onto different drives.
  • To accommodate more than one operating system.

When MS-DOS was first designed to use hard disk drives, the largest hard drive that could be used was 32 MB (because of the way MS-DOS stored files on the hard drive). Partitioning was included in MS-DOS 3.3. This allowed for the development of larger physical hard drives by creating multiple logical drives of up to 32 MB each. Starting with MS-DOS 4.0, the partition size was increased to 512 MB. Beginning with MS-DOS 5.0, the partitions can be as large as 2 GB. Windows 98 and 2000 support much larger drive sizes, and many new disks exceed 20 GB.

Some hard disk drives that exceed 4 GB might not work with an older computer, BIOS, or operating system. They will physically function, but the whole drive cannot be accessed-disk access will be limited to the largest size that can be recognized by that system.

Primary and Extended Partitions

There are two types of partitions: primary and extended. The primary partition is the location where the boot information for the operating system is stored. To boot from a hard disk drive, it must have a primary partition. Primary partitions are for storage of the boot sector, which tells the computer where to find the operating system. The name of the primary partition is C.

The extended partition is for a hard disk drive, or part of a hard disk drive, that does not have an operating system. The extended partition is not associated with a "physical" drive letter. Instead, the extended partition is further divided into logical drives starting with D and progressing until drive letter Z is created. (Remember: A and B are reserved for floppy disk drives.)

Newer operating systems can use all of the drive as a single primary partition. The logical drive concept was invented to allow older versions of MS-DOS and Windows to make use of drives that exceeded their maximum drive size.

The following table provides examples of partitions:

One 500-MB physical drive with one partition:

C (primary drive)

One physical drive and one logical drive

One 1-GB physical drive with two partitions:

C (400-MB primary drive)

D (600-MB extended drive)

One physical drive and two logical drives

One 4.3-GB physical drive with three partitions:

C (1-GB primary drive)

D (1.65-GB extended drive)

E (1.65-GB extended drive)

One physical drive and three logical drives