The CHKDSK command

There must be at least one blank between the drive specifier and the slash 
indicating further options if using <CHKDSK for an HPFS formatted drive. 

Example: 

 
CHKDSK C:/F

will work for FAT formatted drives, but not for HPFS formatted drives. 

 
CHKDSK C: /F

will work for both, FAT formatted and HPFS formatted drives.