Hi all. I've just purchased a 2008 24inch iMac which had Yosemite installed. All three of the rear USB ports were not working at all when I got it. I have since upgraded to El Capitan to see if this would resolve the issue...but alas no. El Capitan is also bringing up the "USB Devices Disabled" notification even though no devices are plugged in. I've tried the usual SMC and PRAM resets to no avail. Next I'm thinking of doing a clean install of El Capitan or even Lion to see if this makes any difference. I do have working Firewire, iSight and Bluetooth. I've read a few posts about USB problems with El Capitan and Yosemite so hoping this is not hardware failure.