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.