freebsd 5.3
$ man dd
...
     conv=value[,value ...]
               Where value is one of the symbols from the following list.
...
               noerror  Do not stop processing on an input error.  When an
                        input error occurs, a diagnostic message followed by
                        the current input and output block counts will be writ-
                        ten to the standard error output in the same format as
                        the standard completion message.  If the sync conver-
                        sion is also specified, any missing input data will be
                        replaced with NUL bytes (or with spaces if a block ori-
                        ented conversion value was specified) and processed as
                        a normal input buffer.  If the fillchar option is spec-
                        ified, the fill char provided on the command line will
                        override the automatic selection of fill character.  If
                        the sync conversion is not specified, the input block
                        is omitted from the output.  On input files which are
                        not tapes or pipes, the file offset will be positioned
                        past the block in which the error occurred using
                        lseek(2).
...
               sync     Pad every input block to the input buffer size.  Spaces
                        are used for pad bytes if a block oriented conversion
                        value is specified, otherwise NUL bytes are used.