Galaxy S II GT-I9100G USB connectivity problem
Worked for me:


Unplug the USB cable
On the cell phone, dial the following number: *#7284# Once the last # is hit, the PhoneUtil application is launched. Choose USB -> Modem and then USB -> PDA mode. The good mode should be PDA. Even if the mode is PDA, switch to Modem and then back to PDA.
Plug the USB cable.


Note:


Repair USB connection for any Samsung Android phone


Reference: http://www.youtube.com/watch?v=KV2h5O_qDPE