Increasing microcomputer storage capacities led to their introduction, eventually followed by long filenames.
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. This is often done to differentiate them from local drives: by using letters towards the end, it reduces the risk of an assignment conflict.
This is especially true when the assignment is done automatically across a network (usually by a logon script).
SYS 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.
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.
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).
Windows 9x (MS-DOS 7.0/MS-DOS 7.1) added support for ), but this is not officially supported and may break programs that assume that all drive letters are A: to Z:.
Drive letters are not the only way of accessing different volumes.
MS-DOS on the Apricot PC assigns letters to hard drives, starting with A:, before considering floppy drives.