A "Drive Letter" is a relic left over from the days of CP/M. It follows that, on every Microsoft-based system since then, Alphabetical Characters are used to point to a Disk Volume or Mount Point.
Some Drive Letters are automatically reserved.
"A" is always reserved for the Floppy Drive (However, this does not apply on newer systems without a Floppy Drive).
"B" is always reserved for a second Floppy Drive.
"C" is always reserved for the first Bootable Hard Drive Partition, usually the Primary Partition.
"D" will be used if no futher Partitions are found on the current Hard Drive, for an Optical Drive if one is present, as long as anoter Hard Drive isn't present in the Device Chain before it. So, if the Optical Drive is located before another Hard Drive, it will have a drive letter assigned before the Hard Drive does. Here are some Examples:
Example 1:
- Primary IDE Channel
- Hard Drive
- CD-ROM Drive
- Secondary IDE Channel
- Hard Drive
- Hard Drive
Here, the first Hard Drive on the Primary IDE Channel will be assigned drive letters first, followed by the CD-ROM Drive, and then by the two Hard Drives on the Secondary IDE Channel.
Example 2:
- Primary IDE Channel
- Hard Drive
- Hard Drive
- Secondary IDE Channel
- CD-ROM Drive
- Hard Drive
Here, both Hard Drives on the Primary IDE Channel will have Drive Letters assigned to them first, followed by the two devices, in sequence, on the Secondary IDE Channel.
Drive Letters can also be assigned for USB Mass Storage Devices, or for SCSI Devices.