My late 2012 27" iMac has four USB ports on the back. Currently 2 do not work....most of the time. I had a mouse and a scanner/printer connected to them. I can use both using a USB port on my non-wireless keyboard and I can sometimes -- only sometimes -- use them through the ports on the back. This morning the rear ports worked OK but now after a shut down and restart when I was away during the day neither work. The other two ports work fine. The two ports that do not work are the two closest to the edge. The ones that do work (used for keyboard and Time Machine drive) are the ones closer to the center. What is going on? When I came back from vacation I had red ants crawling through the vents and across the screen (between the screen and the glass -- very strange). Could they have caused a problem? What can I do?