I was looking for the answer to this as well, found this thread but without answer. So I asked Sandisk support, here is their official reply about the 16MB unallocated space :
“You may leave the 16MB unallocated space as it is. A portion of the total capacity is used to store certain functions including optimizations of the memory that support performance and endurance and therefore is not available for user storage.”
So I guess it is best to leave it that way, it may prolong the write cycles of the card.
