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

       /WRITE
       /NOWRITE

    Controls whether the volume can be written. You can specify
    /NOWRITE to protect files by providing read-only access.
    Specifying /NOWRITE is equivalent to write-locking the device.

    The default is /WRITE. If /WRITE is specified (either explicitly
    or by default) and the volume itself is write-locked, EXCHANGE
    displays a message to inform you that the volume is write-locked.