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

    Dismounts only one volume of a volume set on the specified
    device. By default, all volumes in a set are dismounted.

                                   NOTE

       Avoid dismounting the root volume of a volume set, because
       it contains the master file directory (MFD). It may be
       impossible to access files on a volume set if the MFD is
       not accessible.