A common concern with the current crop of Solid State Drives is the performance penalty associated with block-rewriting. The flash memory used on today's SSDs is comprised of cells that usually contain 4KB pages that are arranged in blocks of 512KB. When a cell is unused, data can be written to it relatively quickly. But if a cell already contains some data--no matter how little, even if it fills only a single page in the block--the entire block must be re-written. - Tracy