“ReadyBoost can speed up your computer by using storage space on most USB flash drives and flash memory cards. When you plug a ReadyBoost-compatible storage device into your computer, the AutoPlay dialog box offers you the option to speed up your computer using ReadyBoost. If you select this option, you can choose how much memory on the device to use for this purpose.
When you set up a device to work with ReadyBoost, Windows shows you how much space it recommends you allow it to use for optimal performance. For ReadyBoost to effectively speed up your computer, the flash drive or memory card should have at least 1 gigabyte (GB) of available space. If your device doesn't have enough available space for ReadyBoost, you'll see a message telling you to free some space on the device if you want to use it to speed up your system.
You can enable or disable ReadyBoost for a specific flash drive or other removable storage device. For more information, see Turn ReadyBoost on or off for a storage device.”