 the allocation unit (cluter) in unit of byte. The valid value is N times the sector size. N is power of 2 from 1 to 128 for FAT volume and upto 16MiB for exFAT volume. If zero is given, the default allocation unit size is selected depends on the volume size.</dd>
<dt>work</dt>
<dd>Pointer to the working buffer used for the format process.</dd>
<dt>len</dt>
<dd>Size of the working buffer in unit of byte. It needs to be the sector size at least. Plenty of working buffer reduces number of write transactio