The EXCHANGE command MOUNT enters the description of the foreign
    volume in internal tables maintained by EXCHANGE. This command is
    different from the DCL command MOUNT, which enters the device in
    tables maintained by the OpenVMS operating system.

    A virtual volume must be explicitly mounted with the
    MOUNT/VIRTUAL command.

    If an EXCHANGE command is given on an unmounted foreign volume,
    EXCHANGE attempts to execute an implied MOUNT/FOREIGN/WRITE-
    /NODATACHECK on the device. This feature enables EXCHANGE to
    operate in the single-command DCL mode.

    If a MOUNT/FOREIGN (either implied or explicit) command is given
    for a foreign device that has not been mounted on the OpenVMS
    system, EXCHANGE issues the equivalent of the DCL command
    MOUNT/FOREIGN and attempts to make the volume known to the
    operating system. Any volume mounted in this way remains mounted
    after EXCHANGE exits.

    When EXCHANGE issues the MOUNT/FOREIGN command, the system checks
    the following:

    o  That the device has not been allocated to another user

    o  That a volume is physically loaded on the specified device

    o  For magnetic tapes, the volume accessibility field of the VOL1
       label