Sometimes your PC does not recognize the samsung smartphone when connected via USB. You have tried using different usb ports or different pc's but without any luck. You may try the following as well:
1) Disconnect phone from usb
2) In phone dialpad, dial *#7284#
3) A menu will show up for UART and USB settings. Make sure PDA is selected for USB. If it's already selected you may select MODEM and then go back to select PDA again.
4) Connect your phone to PC via USB