Win7/64 Laptop: Auto-speedup On Flash Drive Always Inserted?

Hi! I have a Win7Pro/64 laptop at a day program, and, as its hard drive is very small, I have in it a 256GB flash drive to act as a second hard drive. When I insert a flash drive, the OS asks what I want to do with the drive, and one of the options is to use the drive to speed up the system. The question is how do I get it to do this to this particular flash drive at startup? A side question: what does this actually do?