SUBST--Notes Using other commands with SUBST The following commands do not work, or should not be used, on drives used in the SUBST command: ASSIGN DISKCOPY RECOVER BACKUP FDISK RESTORE CHKDSK FORMAT SYS DEFRAG LABEL UNDELETE /S DISKCOMP MIRROR Valid drive1 values The drive1 parameter must be within the range specified by the LASTDRIVE command. If not, SUBST displays the following error message: Invalid parameter - drive1: Using SUBST with Microsoft Windows Do not create or delete virtual drives using the SUBST command while Microsoft Windows is running. If you want to create or delete a virtual drive using the SUBST command, quit Windows, create or delete the virtual drive, and then start Windows again. Using SUBST with compressed drives You can use SUBST to associate a path on a compressed drive with a drive letter. However, the drive size and free space of the SUBST drive (for example, as reported by the DIR command) might not match the drive size and free space reported for the compressed drive.