Error Message:

Cannot CHKDSK a SUBSTed or ASSIGNed drive.

Explanation:

A SUBSTed drive is a virtual drive hosted on another volume, and cannot be checked. The host volume must be checked instead. The Chkdsk command does not work on drives formed by the Subst or Assign commands.

User Action:

Determine the host volume of the SUBSTed drive and run Chkdsk on that volume.