Readyboost is a new feature that lets a flash drive become RAM for your computer on this Windows operating system
... follow this link to get an idea ... a USB 
drive to your computer and enable ReadyBoost, Windows will... [DOC] [TLE] Understand ReadyBoost and whether it will Speed Up your System [PAR] This feature uses external USB flash drives as a hard disk cache to improve disk 
... is an SSD, Windows 7 disables ReadyBoost when reading from an SSD drive.