Are you saying that you want to use a hard drive for ReadyBoost to speed up your system?
If so, regular hard disk drives do not have level of performance necessary to use them for ReadyBoost. If you try to do it, Windows will give you a nice error message saying you can't for the reason I just gave.
Just buy a bigger stick of RAM (what ReadyBoost attempts to synthesize) if you're running low on memory. You could try increasing the size of your page file instead, but buying more RAM will give better performance.
Hope that helps.
P.S. In your future posts, could you please provide more than a couple fragmented sentences in your topic? It is very hard to "read between the lines", so to speak, on a forum, especially when you provide extremely little information about what your problem is or what you want to do. If you gave a specific and clear problem statement to start with, it would help the people who attempt to assist you provide advice much quicker without having to ask questions about what you want, ask for clarification, etc. Thanks.