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

    Command Qualifier

    Specifies that the BACKUP ignore any file structure on the volume
    and to process the volume in terms of physical blocks. If you
    write a save set with the BACKUP/PHYSICAL command, you must also
    restore it with the BACKUP/PHYSICAL command.

    For physical copy operations between disks, the output disk must
    be the same type of device as the input disk. For physical save
    operations between disks, the output disk must be the same type
    of disk as the input disk or a larger-capacity disk. For physical
    restore operations between disks, the output disk must be the
    same type of device as the disk from which the save set was
    created.

    For all physical operations, the output disk must not have a bad
    block in any location that corresponds to a good block on the
    input disk. (This restriction does not apply to RA or more recent
    disk architectures.)