It is built into Flash, I have it in my CS3 version. I am not sure if it is available in lower versions.
You can look it up by opening up flash and open the Flash help window pane. From there just search fileReference, and you will see the class and it's methods and properties listed.
It looks like all you can do with it though is allow the user to reference a file from their computer, much like the <input type="file" /> works in HTML.
You may be able to default it to open to the lettered drive that the flash drive is mounted on.