Stop "caching" in on your usb devices.

Even when all windows are closed and all applications that used files on the USB drive have been stopped, the device often continues to be used by the system. If the device contains a large number of small files it may be that the host computer is indexing all of those files for quick searching. Turn off indexing on each USB device the first time you plug it into a computer and that cause will be eliminated.

There is also the file caching that is done automatically. To turn that of, right-click on the drive in "My Computer" > click on the "Hardware" tab > highlight the USB device in question > click "properties" in that box > click "Change Settings" > click on the "policies" tab > select "Optimize for quick removal." > click "OK." (Note, you may need to do this on each USB storage device you attach.)

A third item that MAY cause this is "Ready Boost." Your system automatically tests each new USB device. If it thinks it is suitable, it may decide to use it to "speed up" your machine. You can tell your OS to keep its hands off USB devices using the instructions found here: http://windows.microsoft.com/en-US/windows-vista/Turn-ReadyBoost-on-or-off-for-a-storage-device