So today I upgraded my 8 core Mac Pro 3.1 to Mountain Lion. To my dismay I found my USB ports had stopped working, After shouting, Swearing and wanting to make Apple rue the day they overlooked such an issues I found the solution! Online I saw someone mention about the AirPort card causing the USB ports to stop working. How do you ask? If you've bought your Mac second hand and it has BT+AP Combo there's a chance someone has installed a Atheros Apple wireless G card, While that works fine in SL and under for some reason the older AirPort cards do not work in ML Also if you have had the machine from new and bought a generic AP card on eBay that can cause issues too. If your system was specd with Wifi from new check that it has a Broadcom card in it which supports Wireless N. So try removing your wireless card and see if your USB ports start working...If so then hooray you now need to replace it with a newer card. If not then well...It was worth a try!