VMS Help DISMOUNT, Qualifiers, /OVERRIDE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /OVERRIDE=CHECKS

    Marks a Files-11 volume for dismounting even if files are open
    on the volume. If you specify DISMOUNT/OVERRIDE=CHECKS, the
    DISMOUNT command displays messages indicating any open files or
    other conditions that prevent dismounting, immediately followed
    by a message indicating that the volume has been marked for
    dismounting.

    This command does not close open files on the device. A device
    cannot be properly dismounted until either all processes with
    open files have properly closed them, or the processes have been
    rundown completely.

    A substantial amount of time can pass between the time you enter
    the DISMOUNT/OVERRIDE=CHECKS command and the completion of the
    dismount operation. Always wait for the dismount to complete
    before you remove the volume. (To verify that the dismount
    has completed, enter the SHOW DEVICES command.) Note that the
    final phase of volume dismounting occurs in the file system, and
    all open files on the volume must be closed before the actual
    dismount can be done. Note also that the file system cannot
    dismount a volume while any known file lists associated with
    it contain entries.

    By using this command, the device is marked for dismount. This
    prevents additional processes from opening files on the device
    while existing open files are closed.