Drive Letter Assignment - Order of Assignment

Order of Assignment

Except for CP/M and versions of MS-DOS/PC DOS prior to version 5.0, each of these operating systems assigns drive letters according to the following algorithm:

  1. Assign the drive letter A: to the first floppy disk drive (drive 0), and B: to the second floppy disk drive (drive 1). If only one physical floppy is present, drive B: will be assigned to a phantom floppy drive mapped to the same physical drive and dynamically assigned to either A: or B: for easier floppy file operations. If no physical floppy drive is present, DOS 4.0 will assign both A: and B: to the nonexistent drive, whereas DOS 5.0 and higher will invalidate these drive letters. If more than two physical floppy drives are present DOS versions prior to 5.0 will assign subsequent drive letters, whereas DOS 5.0 and higher will remap these drives to higher drive letters at a later stage; see below.
  2. Assign a drive letter to the first active primary partition recognized upon the first physical hard disk. DOS 5.0 and higher will ensure that this will become drive letter C:, so that the boot drive will either have drive letter A: or C:.
  3. Assign subsequent drive letters to the first primary partition upon each successive physical hard disk drive. DOS versions prior to 5.0 will probe for only two physical harddisks, whereas DOS 5.0 and higher support eight physical harddisks.
  4. Assign subsequent drive letters to every recognized logical partition present in the first extended partition, beginning with the first hard drive and proceeding through successive physical hard disk drives.
  5. DOS 5.0 and higher: Assign drive letters to all remaining primary partitions, beginning with the first hard drive and proceeding through successive physical hard disk drives.
  6. DOS 5.0 and higher: Assign drive letters to all physical floppy drives beyond the second physical floppy drive.
  7. Assign subsequent drive letters to any block device drivers loaded in CONFIG.SYS via DEVICE statements, f.e. RAM Disks.
  8. Assign subsequent drive letters to any dynamically loaded drives via CONFIG.SYS INSTALL statements, in AUTOEXEC.BAT or later, f.e. additional optical disc drives (MSCDEX), PCMCIA / PC Card drives, USB or Firewire drives, or network drives.

MS-DOS/PC DOS versions 4.0 and earlier assign letters to all of the floppy drives before considering hard drives, so a system with four floppy drives would call the first hard drive E:. Starting with DOS 5.0, the system ensures that drive C: is always a hard disk, even if the system has more than two physical floppy drives.

While without deliberate remapping the drive letter assignments are typically fixed until the next reboot, Zenith MS-DOS 3.21 will update the drive letter assignments when resetting a drive. This may cause drive letters to change without reboot if harddisk partitioning was changed.

MS-DOS on the Apricot PC assigns letters to hard drives, starting with A:, before considering floppy drives. A system with two of each drive would call the hard drives A: and B:, and the floppies C: and D:.

Some versions of DOS don't assign the drive letter, beginning with C:, to the first active primary partition recognized upon the first physical hard disk, but on the first primary partition recognized of the first hard disk, even if it is not set active.

If there is more than one extended partition in a partition table, only the logical drives in the first recognized extended partition type are processed.

Some late versions of the DR-DOS IBMBIO.COM provide a pre-boot config structure, holding bit flags to select (beside others) between various drive letter assignment strategies. These strategies can be pre-selected by a user or OEM or be changed by a boot loader on the fly when launching DR-DOS. Under these issues, the boot drive can be different from A: or C: as well.

The drive letter order can depend on whether a given disk is managed by a boot-time driver or by a dynamically loaded driver. For example, if the second or third hard disk is of SCSI type and on MS-DOS requires drivers loaded through the CONFIG.SYS file (e.g. the controller card does not offer on-board BIOS or using this BIOS is not practical), then the first SCSI primary partition will appear after all the IDE partitions on MS-DOS. Therefore MS-DOS and, for example, OS/2 could have different drive letters, as OS/2 loads the SCSI driver earlier. A solution was not to use primary partitions on such hard disks.

In Windows NT, Windows 2000, Windows XP and OS/2, the operating system uses the aforementioned algorithm to automatically assign letters to floppy disk drives, optical disc drives, the boot disk, and other recognized volumes that are not otherwise created by an administrator within the operating system. Volumes that are created within the operating system are manually specified, and some of the automatic drive letters can be changed. Unrecognized volumes are not assigned letters, and are usually left untouched by the operating system.

A common problem that occurs with the drive letter assignment is that the letter assigned to a network drive can interfere with the letter of a local volume (like a newly installed CD/DVD drive or a USB stick). For example, if the last local drive is drive D: and we have assigned to a network drive as E:, then when we connect a USB mass storage device it will also be assigned drive E: causing loss of connectivity with either the network share or the USB device. Users with administrative privileges can assign drive letters manually to overcome this problem.

Another condition that can cause problems on Windows XP is when there are network drives defined but in an error condition (as they would be on a laptop operating outside the network). Even when the unconnected network drive is not the next available drive letter, Windows XP may be unable to map a drive and this error may also prevent the mounting of the USB device.

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