Max Logical Drives in MS-DOS 6.22

MS-DOS 6.22 can address a maximum of 26 logical drives simultaneously. This limitation is fundamentally tied to the operating system's design, which assigns individual drive volumes to single letters of the English alphabet ranging from A to Z. Achieving this maximum capacity requires configuring system settings properly, as default installations typically support fewer drives out of the box.

The drive-lettering scheme in MS-DOS begins with reserved letters for specific media types. Drives A: and B: are reserved for floppy disk drives. Even if a system contains only a single physical floppy drive, B: remains reserved as a phantom or virtual drive mapped to the same hardware. Drive C: is allocated to the primary partition of the first physical hard disk.

The remaining letters, D: through Z:, are available for additional logical drives. These letters can represent logical drives within extended partitions, additional physical hard drives, RAM drives (created via RAMDRIVE.SYS), optical drives (accessed through MSCDEX.EXE), or mapped network volumes via network redirectors like Microsoft Network Client.

To enable MS-DOS 6.22 to access the maximum 26 drives, the CONFIG.SYS file must contain the following directive:

LASTDRIVE=Z

By default, if the LASTDRIVE command is omitted from CONFIG.SYS, MS-DOS sets the last available drive letter to one letter past the last physical drive detected during boot, or defaults to E:. Each drive letter specified by LASTDRIVE consumes a small amount of conventional memory (approximately 80 to 90 bytes per drive letter structure) to maintain internal Current Directory Structures (CDS).

Because MS-DOS 6.22 uses the FAT16 file system, individual partitions and logical drives are capped at a maximum size of 2 gigabytes (GB) each, utilizing 32-kilobyte clusters. Consequently, while the operating system can mount up to 26 logical drives simultaneously, the maximum theoretically addressable storage capacity across all non-floppy drives (C: through Z:, totaling 24 drives) is 48 GB.