XPS 8700 8.1

When a device is plugged into the front 2 USB ports on PC, the device is recognized but without an alert or sound coming from PC. This works perfectly on all of the 6 ports on rear of machine and the 2 on top, just not on the front. Dell had a tech swap out the front 2 USB ports but the trouble still existed. After many calls and hours trying everything remotely, Dell sent me a new PC The trouble is the same on this new PC. Actually the new PC did give off an alert the first couple of attempts plugging in different devices, but then after 2 tries, it ceased to give the alert. The devices I am plugging in are: Seagate external drive, Simple Tech Signature Mini external drive, Canon EOS T4I Camera, San Disk Flash Drives. Hope this clarifies the issue for you. I am not that knowledgeable about computer troubleshooting but would very much appreciate any input or information to rectify this problem for me. Thank you for your assistance. Richard Stadler (Case Number <ADMIN NOTE: Case number removed per privacy policy>)