 NAND flash memory forms the core of the removable USB storage devices known as USB flash drives, as well as most memory card formats and solid-state drives available today.The architecture of NAND Flash means that data can be read and programmed in pages, typically between 4 KiB and 16 KiB in size, but can only be erased at the level of entire blocks consisting of multiple pages and MB in size.When a block is erased all the cells are logically set to 1.