Drive letter assignment

In computer data storage, drive letter assignment is the process of assigning alphabetical identifiers to volumes. Unlike the concept of UNIX mount points, where volumes are named and located arbitrarily in a single hierarchical namespace, drive letter assignment allows multiple highest-level namespaces. Drive letter assignment is thus a process of using letters to name the roots of the "forest" representing the file system; each volume holds an independent "tree" (or, for non-hierarchical file systems, an independent list of files).
Origin.
The concept of drive letters, as used today, presumably owes its origins to IBM's VM family of operating systems, dating back to CP/CMS in 1967 (and its research predecessor CP-40), by way of Digital Research's (DRI) CP/M. The concept evolved through several steps:
The important capability of hierarchical directories within each drive letter was initially absent from these systems. This was a major feature of UNIX and other similar operating systems, where hard disk drives held thousands (rather than tens or hundreds) of files. Increasing microcomputer storage capacities led to their introduction, eventually followed by long filenames. In file systems lacking such naming mechanisms, drive letter assignment proved a useful, simple organizing principle.
Order of assignment.
MS-DOS/PC DOS since version 5.0, and later operating systems, assigns drive letters according to the following algorithm:
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, however, 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 the partitioning of the harddisk 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 do not 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 preboot config structure, holding bit flags to select (beside others) between various drive letter assignment strategies. These strategies can be preselected 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 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 DOS. Therefore, 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 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 a network drive would have been assigned as E:, then a newly attached USB mass storage device would 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.
Common assignments.
Applying the scheme discussed above on a fairly modern Windows-based system typically results in the following drive letter assignments:
Case-specific drive letters:
When there is no second physical floppy drive, drive B: can be used as a "virtual" floppy drive mapped onto the physical drive A:, whereby the user would be prompted to switch floppies every time a read or write was required to whichever was the least recently used of A: or B:. This allows for much of the functionality of two floppy drives on a computer that has only one. This concept of multiple drive letters sharing a single physical device (optionally with different "views" of it) is not limited to the first floppy drive, but can be utilized for other drives as well by setting up additional block devices for them with the standard DOS DRIVER.SYS in CONFIG.SYS.
Network drives are often assigned letters towards the end of the alphabet. This is often done to differentiate them from local drives: by using letters towards the end, it reduces the risk of an assignment conflict. It is especially true when the assignment is done automatically across a network (usually by a logon script).
In most DOS systems, it is not possible to have more than 26 mounted drives. Atari GEMDOS supports 16 drive letters A: to P: only. The PalmDOS PCMCIA driver stack supports drive letters 0:, 1:, 2:, ... to address PCMCIA drive slots.
Some Novell network drivers for DOS support up to 32 drive letters under compatible DOS versions. In addition, Novell DOS 7, OpenDOS 7.01, and DR-DOS 7.02 genuinely support a CONFIG.SYS codice_5 directive in order to allocate up to 32 drive letters, named A: to Z:, [:, \:, ]:, ^:, _: and `:. (DR-DOS 7.02-7.07 also supports codice_6 and codice_7 directives in order to relocate drive structures into upper memory.) Some DOS application programs do not expect drive letters beyond Z: and will not work with them, therefore it is recommended to use them for special purposes or search drives.
JP Software's 4DOS command line processor supports drive letters beyond Z: in general, but since some of the letters clash with syntactical extensions of this command line processor, they need to be escaped in order to use them as drive letters.
Windows 9x (MS-DOS 7.0/MS-DOS 7.1) added support for codice_5 and codice_9 as well.
If access to more filesystems than Z: is required under Windows NT, Volume Mount Points must be used. However, it is possible to mount non-letter drives, such as 1:, 2:, or !: using the command line codice_10 utility in Windows XP or later (i.e. codice_11), but it is not officially supported and may break programs that assume that all drives are letters A: to Z:.
ASSIGN, JOIN and SUBST in DOS and Windows.
Drive letters are not the only way of accessing different volumes. DOS offers a JOIN command that allows access to an assigned volume through an arbitrary directory, similar to the Unix "mount" command. It also offers a SUBST command which allows the assignment of a drive letter to a directory. One or both of these commands were removed in later systems like OS/2 or Windows NT, but starting with Windows 2000, both are again supported: The SUBST command exists as before, while JOIN's functionality is subsumed in LINKD (part of the Windows Resource Kit). In Windows Vista, the new command MKLINK can be used for this purpose. Also, Windows 2000 and later support mount points, accessible from the Control Panel.
Many operating systems originating from Digital Research provide means to implicitly assign substitute drives, called "floating drives" in DRI terminology, by using the CD/CHDIR command in the following syntax:
 CD N:=C:\SUBDIR
DOS Plus supports this for drive letters N:, O:, and P:. This feature is also present in Concurrent DOS, Multiuser DOS, System Manager 7, and REAL/32, however, these systems extend the concept to all unused drive letters from A: to Z:, except for the reserved drive letter L:. DR DOS 3.31 - 6.0 (up to the 1992-11 updates with BDOS 6.7 only) also supports this including drive letter L:. This feature is not available under DR DOS 6.0 (1992 upgrade), PalmDOS 1.0, Novell DOS 7, OpenDOS 7.01, DR-DOS 7.02 and higher. Floating drives are implemented in the BDOS kernel, not in the command line shell, thus they can be used and assigned also from within applications when they use the "change directory" system call. However, most DOS applications are not aware of this extension and will consequently discard such directory paths as invalid. JP Software's command line interpreter 4DOS supports floating drives on operating systems also supporting it.
In a similar feature, Concurrent DOS, Multiuser DOS, System Manager and REAL/32 will dynamically assign a drive letter L: to the load path of a loaded application, thereby allowing applications to refer to files residing in their load directory under a standardized drive letter instead of under an absolute path. This load drive feature makes it easier to move software installations on and across disks without having to adapt paths to overlays, configuration files or user data stored in the load directory or subsequent directories.
In some versions of DR-DOS, the load path contained in the appendage to the environment passed to drivers can be shortened to that of a temporary substitute drive (e.g. codice_12) through the codice_13/codice_14 option codice_15 (for codice_16 instead of, say, codice_17). This can be used to minimize a driver's effective memory footprint, if the executable is located in a deep subdirectory and the resident driver happens to not need its load path after installation any more.