How MS-DOS 6.22 Assigns Secondary Hard Drive Letters

In MS-DOS 6.22, drive letter assignment follows a strict, hardware-level hierarchy based on partition types across physical drives rather than reading each drive completely in order. When a secondary hard disk is installed, the operating system assigns drive letters by prioritizing primary partitions on all physical drives before moving on to logical drives within extended partitions. This article explains the exact sequence MS-DOS 6.22 uses to allocate letters to secondary hard disks and how this process affects your existing drive configuration.

The Standard MS-DOS Enumeration Order

MS-DOS scans the system's storage controllers during boot and allocates drive letters alphabetically using the following sequence:

  1. Floppy Drives: Reserved for A: and B:. If only one physical floppy drive exists, MS-DOS still reserves B: as a logical phantom drive for disk duplication.
  2. Primary DOS Partition on Disk 1: Assigned to C:.
  3. Primary DOS Partitions on Secondary Disks: The primary DOS partition of the second physical hard disk is assigned D:. If a third or fourth physical drive exists with a primary DOS partition, they receive E:, F:, and so on.
  4. Logical Drives on Disk 1: MS-DOS returns to the first physical disk to assign letters to all logical drives found inside its Extended DOS partition.
  5. Logical Drives on Secondary Disks: MS-DOS proceeds to the secondary hard disk (and subsequent drives) to assign letters to any logical drives inside its Extended DOS partition.
  6. Device Drivers: Block devices defined in CONFIG.SYS (such as RAMDRIVE.SYS or CD-ROM drives via MSCDEX) receive the next available letters.

How Secondary Hard Disks Are Partitioned

The drive letters your secondary hard disk receives depend entirely on how you partition it using FDISK:

  • With a Primary Partition: If the secondary disk contains a Primary DOS partition, that partition immediately takes the D: drive letter. Any logical drives inside an Extended DOS partition on the first disk are pushed down the alphabet (e.g., what was previously D: on the first disk becomes E:).
  • Without a Primary Partition: If the secondary disk contains only an Extended DOS partition with logical drives, MS-DOS completes assigning all letters on the primary hard disk first. The logical drives on the secondary disk then follow consecutively after the primary disk's logical drives, preventing any drive letter shifting on the original drive.

The Drive Letter Shifting Effect

The most critical aspect of adding a secondary hard disk in MS-DOS 6.22 is drive shifting. If your original hard drive has partitions C: (Primary) and D: (Logical), adding a second physical drive with its own Primary DOS partition alters the map:

  • Primary Hard Disk (Primary Partition): Remains C:
  • Secondary Hard Disk (Primary Partition): Becomes D:
  • Primary Hard Disk (Logical Drive): Shifts from D: to E:
  • Secondary Hard Disk (Logical Drive): Becomes F:

To prevent broken file paths and shortcuts when installing a secondary hard disk, configure the secondary drive solely as an Extended DOS partition containing logical drives, skipping the creation of a Primary DOS partition entirely.