MS-DOS 6.22: ScanDisk vs CHKDSK Differences

When Microsoft released MS-DOS 6.22, it positioned the SCANDISK utility as the modern successor to the aging CHKDSK (Check Disk) command. While both tools were designed to maintain file system integrity on FAT-formatted media, SCANDISK introduced an interactive graphical interface, physical surface scanning, safer automated repairs, and deep integration with DriveSpace compressed drives. Unlike CHKDSK, which offered rudimentary, command-line-only diagnostics with limited repair options, SCANDISK provided a comprehensive disk diagnostic suite that significantly reduced the risk of data loss.

User Interface and Interactivity

CHKDSK is strictly a command-line tool. When run, it outputs a brief text summary detailing total disk space, hidden files, directories, user files, and available conventional memory. If errors were detected, users had to rerun the command with the /F (Fix) parameter, which made changes automatically without previewing the proposed fixes.

SCANDISK introduced a full-screen, text-mode graphical user interface (TUI). It provided visual progress bars, detailed descriptions of detected anomalies, and interactive prompts asking the user how to handle each problem. Although it could be automated with command-line switches (such as /AUTOFIX or /CHECKONLY), its default interactive mode made disk diagnosis much more accessible to everyday users.

Logical Analysis vs. Physical Surface Scanning

CHKDSK checks only the logical structure of a disk. It inspects the File Allocation Table (FAT), directory structures, and file size consistency, but it does not verify whether the underlying magnetic media can reliably store data.

SCANDISK performs all the logical checks of CHKDSK and adds a dedicated surface scan. During a surface scan, SCANDISK reads every cluster on the drive to detect physical defects. If it encounters a damaged or weak sector, it attempts to read the data, relocates that data to a healthy sector, and marks the damaged cluster in the FAT as "bad" to prevent the operating system from writing to it in the future.

Safer Error Handling and Undo Capabilities

When CHKDSK fixes disk errors using the /F switch, it uses an aggressive, all-or-nothing approach:

  • Lost Allocation Units: CHKDSK converts orphaned clusters into sequential files named FILE0000.CHK in the root directory, often cluttering the disk without clarifying what the data originally was.
  • Cross-Linked Files: If two files claim ownership of the same cluster, CHKDSK typically leaves the duplicate link intact or truncates the file, frequently causing permanent corruption.

SCANDISK introduced far more granular and safer recovery routines:

  • Interactive Choices: Users can choose whether to convert lost clusters into files or delete them entirely to free up space.
  • Cross-Link Resolution: SCANDISK provides options to make copies of the cross-linked cluster for each affected file, truncate one of the files, or delete the damaged entry.
  • Undo Disks: Before writing any changes to the FAT, boot sector, or root directory, SCANDISK can save the original structural data to a floppy disk as an "Undo Disk," allowing users to reverse changes if the repair results in data corruption.

Support for Compressed Drives (DriveSpace)

MS-DOS 6.22 relies on DriveSpace for real-time disk compression, which stores an entire compressed file system inside a single, hidden Compressed Volume File (CVF).

CHKDSK has limited visibility into CVF architecture and can only verify the uncompressed host drive or view the CVF as a standard static file. SCANDISK, by contrast, was built with direct awareness of DriveSpace and DoubleSpace volumes. It inspects both the underlying host drive and the internal file allocation structures, header records, and signature blocks inside the CVF itself.

Deprecation of CHKDSK's Repair Functions

In MS-DOS 6.22, running CHKDSK /F displays a prompt warning that SCANDISK is the preferred tool for finding and fixing disk problems. While CHKDSK remained included for backwards compatibility, batch scripts, and rapid reporting of conventional memory usage, Microsoft officially relegated all primary disk analysis and repair operations to SCANDISK.