/sys$common/syshlp/helplib.hlb EXCHANGE, MOUNT, Qualifiers, /DATA_CHECK  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /DATA_CHECK[=(READ,WRITE)]
       /NODATA_CHECK

    Determines whether EXCHANGE performs a second operation after
    every I/O operation to verify that the data was correctly
    transferred. If you specify /DATA_CHECK=WRITE, after every
    write operation EXCHANGE rereads the data that was just written
    and compares it with the original data. If you specify /DATA_
    CHECK=READ, EXCHANGE reads each block of data twice and verifies
    that both read operations received identical data.

    It is usually more efficient to use the /DATA_CHECK option on
    the DCL command MOUNT than to use the option on the EXCHANGE
    command MOUNT. If you mount a device with the DCL command
    MOUNT/FOREIGN/DATA_CHECK, OpenVMS can use features in the
    device hardware and device driver to perform the redundant I/O
    operations.

    The RX01 and RX02 diskette drives do not contain the necessary
    features for the operating system to perform data checking. If
    you use the DCL command MOUNT/DATA_CHECK with a diskette, the
    system is unable to perform data checking (no warning message
    is displayed). EXCHANGE is able to recognize, however, that a
    diskette was mounted with the data checking option; in this case,
    EXCHANGE performs the software data checking internally, even if
    you have not specified an explicit MOUNT/DATA_CHECK command.

    If you specify the /DATA_CHECK qualifier without an option, the
    default is /DATA_CHECK=WRITE.