Thanks for your help, Seve(and also for the other post of mine).
However, what do you mean by "(System/Preferences/Removable...) if it clears up"? I checked it out and to me, everything seemed to be configured the way it should be. It's quite late now, I should get to bed. After I shut down the computer, I will connect the usb cords to different ports and will see if it solves the problem tomorrow morning when I boot up the computer.