OK, after facing similar problem, I have called the store I bought my Samsung Ace 2, and the sale lady gave me this solution and it worked perfectly:
1) Unplug everything.
2) Go to your phone 'setting' -> 'wireless and network' -> 'usb utilities' then press the button 'connect storage to PC' THEN plug in the cable to the phone & computer. Then you will see android logo and it will say USB connected. Then your computer will detect the driver. You will have to repeat these steps every time you mount your usb driver.
Hope this helps. I haven't seen any solution on any forum sofar. If it works for you, please report back too.