PC Hardware in a Nutshell, 3rd Edition

How drive letters are assigned

Partitioning a disk with DOS or Windows assigns a drive letter to each primary partition and each logical volume created in an extended partition, a process called dynamic drive letter assignment. Adding and partitioning another hard disk or changing partitions on an existing hard disk may cause existing drive letters to change, which can confuse programs that expect themselves or their data to be on a particular volume and suddenly find that is no longer true. Modifying partitions under DOS or Windows 9X assigns drive letters as follows:

• Physical disks are numbered sequentially. The Primary Master is Disk 0, the Primary Slave is Disk 1, the Secondary Master is Disk 2, and the Secondary Slave is Disk 3. For SCSI disks, the drive with the lowest SCSI ID is Disk 0, the one with the next-higher SCSI ID is Disk 1, and so on.

• The first primary partition on each physical disk, beginning with Disk 0, is assigned a drive letter sequentially, beginning with C:. The first primary partition on each higher-numbered disk is then assigned the next drive letter in sequence. A disk that has no primary partition is skipped during this process.

• Starting with Disk 0, each logical volume is assigned a drive letter. All logical volumes on each disk are assigned drive letters before drive letters are assigned to any logical volumes on higher-numbered disks.

• Once all logical volumes on all disks have been assigned drive letters, the remaining primary partitions on each disk ...