Hard Disk : How the OS Assigns Drive Letters

These OSs assign drive letters in a fixed sequence which cannot be changed. This
sequence is as follows:
• The OS begins by assigning a drive letter to the first primary partition that it
recognizes on the first system hard disk. The OS then assigns drive letters to the first
primary partition recognized on each successive hard disk. For example, imagine you
have three hard disks in your system. When you boot your OS, it assigns drive letter
C: to the active primary partition on the first hard disk. Drive letter D: is assigned to
the first primary partition that the OS recognizes on the second hard disk, and drive
letter E: is likewise assigned to the first primary partition on the third disk.
If you have multiple visible primary partitions on a single hard disk, the OS assigns
the drive letter to the active partition. If none of the partitions are active, the drive
letter is assigned to the first visible primary partition recognized by the OS.
• Next, all logical partitions recognized by the OS are assigned drive letters, starting
with the logical partitions on the first hard disk and proceeding in order. For example,
suppose you have two hard disks in your system, each with one primary and two
logical partitions. The OS first assigns C: and D: to the two primary partitions, then
assigns drive letters E: and F: to the first and second logical partitions on the first hard
disk. Drive letters G: and H: are assigned to the two logical partitions on the second
disk.The OS then assigns drive letters to any remaining visible primary partitions, starting
with those on the first hard disk. The OS proceeds to any visible primary partitions on
the second disk, then the third disk, and so on.
• Finally, CD-ROM drives and other types of removable media are assigned a drive
letter.
Because the OS always follows this sequence to assign drive letters, adding or removing a
second hard disk can cause changes to your drive letter assignments. Likewise, drive
letters can change if you add, remove, or copy a disk partition; reformat a partition with a
different file system; or boot a different OS.