Write operation and garbage collection methods are provided for a Solid State Drive (SSD) controller of a SSD having Not-AND (NAND) flash memory devices with on-die Static Random Access Memory (SRAM) and NAND flash memory. In the write operation method, a received block of data is stored in on-die SRAM of the NAND flash device, rather than in on-chip SRAM of the controller, prior to programming into NAND flash memory. Until programmed into NAND flash memory, the block of data remains available in the on-die SRAM to fulfill an ‘immediate read’ operation, if received. In the garbage collection method, blocks of data are read from one or more source NAND flash devices and stored in on-die SRAM of a destination NAND flash device until a limit of such blocks has been reached, then the destination NAND flash device programs the blocks from the on-die SRAM into NAND flash memory.