This could also be if you chose to use part of the drive as readyboost.
Here is an address to show you how to remove readyboost from your drive. Its also a good thing to check to make sure, just in case.
From one of the other blogs, I decided to use the HP USB Disk Storage Format Tool. It worked and reformatted the entire free space, in seconds. What I do not understand is why the SANDisk and Windows would not allow me to do that.