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Order in Which MS-DOS and Windows Assign Drive Letters

This article was previously published under Q51978
SUMMARY
Microsoft MS-DOS assigns drive letters to the first two physicalfloppy disk drives and hard disk drives it finds at boot time in afixed sequence, including multiple partitions and logical drives onthe hard disks. You cannot change this sequence.

The drive letters assigned to additional drives installed usingDRIVER.SYS and other installable device drivers is dependent upon theorder in which the drivers are loaded in the CONFIG.SYS file. Thesedrive letter assignments can be influenced by changing the order ofthe CONFIG.SYS statements or loading "dummy" drives to "use up" driveletters.

Drive letter assignments can change when you upgrade from one MicrosoftMS-DOS version to another or from an original equipment manufacturer (OEM)version of MS-DOS to another version that assigns drive lettersdifferently. (The order in which drive letters are assigned was modified byOEMs in earlier versions of MS-DOS.) This article describes how MS-DOSassigns drive letters; it does not explain how particular OEM MS-DOSversions assign drive letters.
MORE INFORMATION
The following occurs at startup:
  1. MS-DOS checks all installed disk devices, assigning the drive letter A to the first physical floppy disk drive that is found.
  2. If a second physical floppy disk drive is present, it is assigned drive letter B. If it is not present, a logical drive B is created that uses the first physical floppy disk drive.
  3. Regardless of whether a second floppy disk drive is present, MS-DOS then assigns the drive letter C to the primary MS-DOS partition on the first physical hard disk, and then goes on to check for a second hard disk.
  4. If a second physical hard disk is found, and a primary partition exists on the second physical drive, the primary MS-DOS partition on the second physical hard drive is assigned the letter D. MS-DOS version 5.0, which supports up to eight physical drives, will continue to search for more physical hard disk drives at this point. For example, if a third physical hard disk is found, and a primary partition exists on the third physical drive, the primary MS-DOS partition on the third physical hard drive is assigned the letter E.
  5. MS-DOS returns to the first physical hard disk drive and assigns drive letters to any additional logical drives (in extended MS-DOS partitions) on that drive in sequence.
  6. MS-DOS repeats this process for the second physical hard disk drive, if present. MS-DOS 5.0 will repeat this process for up to eight physical hard drives, if present. After all logical drives (in extended MS-DOS partitions) have been assigned drive letters, MS-DOS 5.0 returns to the first physical drive and assigns drive letters to any other primary MS-DOS partitions that exist, then searches other physical drives for additional primary MS-DOS partitions. This support for multiple primary MS-DOS partitions was added to version 5.0 for backward compatibility with the previous OEM MS-DOS versions that support multiple primary partitions.
  7. After all logical drives on the hard disk(s) have been assigned drive letters, drive letters are assigned to drives installed using DRIVER.SYS or created using RAMDRIVE.SYS in the order in which the drivers are loaded in the CONFIG.SYS file. Which drive letters are assigned to which devices can be influenced by changing the order of the device drivers or, if necessary, by creating "dummy" drive letters with DRIVER.SYS.
The MS-DOS utility SUBST, networks and programs such as the CD-ROMExtensions which use the MS-DOS network interface can request aspecific drive letter be assigned to a block device.

Example 1

Consider as an example a system with one floppy disk drive and onehard disk drive, with two MS-DOS partitions (a primary partition andan extended partition containing a single logical drive) on the harddisk. In this configuration, MS-DOS will assign the floppy disk driveas drives A and B, the primary partition on the hard disk drive asdrive C, and the logical drive in the extended partition as drive D.

Example 2

Consider another system with three floppy disk drives, the third drivebeing installed using DRIVER.SYS, and two hard disk drives, with aprimary and an extended partition on each hard disk drive. Theextended partition on the first hard disk drive contains two logicaldrives, and the extended MS-DOS partition on the second hard diskdrive contains one logical drive. A RAM disk is also created usingRAMDRIVE.SYS.

In this configuration, MS-DOS will assign the first two floppy diskdrives as drives A and B, then assign the primary partitions on thefirst and second physical hard disk drives as drives C and D,respectively. MS-DOS will then assign the drive letters E and F to thetwo logical drives in the extended partition on the first physicaldrive, and G to the logical drive in the extended partition on thesecond physical drive.

The third floppy disk drive, installed using DRIVER.SYS, and the RAMdisk created using RAMDRIVE.SYS, will be assigned the letters H and Iin the order in which the DEVICE= statements appear in the CONFIG.SYSfile.

Partitioning Schemes

Listed below are some sample partitioning schemes for two 40-megabyte (MB)hard disk drives and their resulting drive letter assignments:
  • Drive 1:
    C: 20 MB primary MS-DOS partition
    E: 20 MB logical drive 1 in extended MS-DOS partition

    Drive 2:
    D: 20 MB primary MS-DOS partition
    F: 20 MB logical drive 1 in extended MS-DOS partition
  • Drive 1:
    C: 20 MB primary MS-DOS partition
    D: 20 MB logical drive 1 in extended MS-DOS partition

    Drive 2:
    E: 20 MB logical drive 1 in extended MS-DOS partition
    F: 20 MB logical drive 2 in extended MS-DOS partition
  • Drive 1:
    C: 10 MB primary MS-DOS partition
    E: 10 MB logical drive 1 in extended MS-DOS partition
    F: 10 MB logical drive 2 in extended MS-DOS partition
    G: 10 MB logical drive 3 in extended MS-DOS partition

    Drive 2:
    D: 10 MB primary MS-DOS partition
    H: 10 MB logical drive 1 in extended MS-DOS partition
    I: 10 MB logical drive 2 in extended MS-DOS partition
    J: 10 MB logical drive 3 in extended MS-DOS partition
6.22 3.20 3.30 3.30a 4.00 4.00a 5.00 5.00a 6.00 6.20 setup upgrade fdisk msdos
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Article ID: 51978 - Last Review: 01/18/2007 19:09:27 - Revision: 2.3

  • Microsoft MS-DOS 3.1
  • Microsoft MS-DOS 3.2 Standard Edition
  • Microsoft MS-DOS 3.21 Standard Edition
  • Microsoft MS-DOS 3.3 Standard Edition
  • Microsoft MS-DOS 3.3a
  • Microsoft MS-DOS 4.0 Standard Edition
  • Microsoft MS-DOS 4.01 Standard Edition
  • Microsoft MS-DOS 5.0 Standard Edition
  • Microsoft MS-DOS 5.0a
  • Microsoft MS-DOS 6.0 Standard Edition
  • Microsoft MS-DOS 6.2 Standard Edition
  • Microsoft MS-DOS 6.21 Standard Edition
  • Microsoft MS-DOS 6.22 Standard Edition
  • Microsoft Windows 95
  • Microsoft Windows 98 Standard Edition
  • Microsoft Windows Millennium Edition
  • KB51978
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