Using ReadyBoost-capable flash memory (NAND memory devices) for caching allows Windows Vista and later to service random disk reads with better performance than without the cache.
ReadyBoost compresses and encrypts, with AES-128, all data that is placed on the flash device; Microsoft has stated that a 2:1 compression ratio is typical, so a 4 GB cache would usually contain 8 GB of data [REF].