/FILL_FACTOR=percentage

    This qualifier is only valid in combination with /COMPRESS.
    Default behaviour for DFU is to compress a directory as tight
    as possible; this is equivalent to /FILL_FACTOR=100.
    By choosing a lower fill_factor DFU will leave some free
    space in each directory block. /FILL_FACTOR may be between
    50 and 100 %. Caution : choosing a fill_factor lower than
    100% may fail if the directory file is not large enough.
    In that case DFU will signal an error and advise using
    a higher fill_factor.