This is the explanation of the CHKDSK command. It is EXTERNAL.

This command will either create or display a status report for
disk. It is helpful to use if you have a problem with a disk.

Examples:
        If you would like to check the disk i drive A and have
        MS-DOS fix any errors it finds you would type:
           chkdsk a: /f

        To do the same thing as above but showing each directory 
        and file as it is checked you type:
           chkdsk a: /v