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

       /BLOCKSIZE=n

    Specifies the default block size for magnetic tape volumes.

    The parameter, n, specifies the default block size value for
    magnetic tape volumes. Valid values are in the range 20 to 65,532
    for OpenVMS RMS operations, and 18 to 65,534 for non OpenVMS
    RMS operations. By default, records are written to magnetic tape
    volumes in 2048-byte blocks. For foreign or unlabeled magnetic
    tapes, the default is 512 bytes.

    You must specify /BLOCKSIZE in two situations:

    o  When mounting magnetic tapes that do not have HDR2 labels.
       For these magnetic tapes, you must specify the block size.
       For example, you must specify /BLOCKSIZE=512 to mount an RT-11
       magnetic tape.

    o  When mounting magnetic tapes that contain blocks whose sizes
       exceed the default block size (2048 bytes). In this case,
       specify the size of the largest block for the block size.

    Example

    In the following example, the /BLOCKSIZE qualifier specifies a
    block size of 1000 bytes; the default for a magnetic tape mounted
    with the /FOREIGN qualifier is 512.

    $ MOUNT/FOREIGN/BLOCKSIZE=1000 MTA1: