/FILL_FACTOR=percentage This qualifier is only valid in
    combination with /COMPRESS. Default behavior for DFU is to
    compress a directory as tight as possible; this is equivalent
    to /FILL_FACTOR=100. By choosing a lower fill factor value DFU
    will leave some free space in each directory block. /FILL_FACTOR
    may be between 50 and 100 %. Caution : choosing a fill factor
    value 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 value.