Order in Which MS-DOS and Windows Assign Drive Letters

Microsoft MS-DOS assigns drive letters to the first two physical floppy disk drives and hard disk drives it finds at boot time in a fixed sequence, including multiple partitions and logical drives on the hard disks. You cannot change this sequence.

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

Drive letter assignments can change when you upgrade from one Microsoft MS-DOS version to another or from an original equipment manufacturer (OEM) version of MS-DOS to another version that assigns drive letters differently. (The order in which drive letters are assigned was modified by OEMs in earlier versions of MS-DOS.) This article describes how MS-DOS assigns drive letters; it does not explain how particular OEM MS-DOS versions assign drive letters.

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-ROM Extensions which use the MS-DOS network interface can request a specific drive letter be assigned to a block device.

Example 1

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

Example 2

Consider another system with three floppy disk drives, the third drive being installed using DRIVER.SYS, and two hard disk drives, with a primary and an extended partition on each hard disk drive. The extended partition on the first hard disk drive contains two logical drives, and the extended MS-DOS partition on the second hard disk drive contains one logical drive. A RAM disk is also created using RAMDRIVE.SYS.

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

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

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