I have 2 3.0 usb ports and 2 2.0 usb ports in the front. All of them stopped working for some strange reason. I tried using them to boot off a linux usb and it doesn't work. Strangely, if I have 2 USB ports occupied in the front and I plug another device into one of the remaining 2 USB ports in the front, that USB device starts working. The front USB ports don't work in safe mode either.