I don't know if you've seen this thread but the solution in 'the solution is so easy by jaybeef - 10/17/05 1:00 PM ' post seems to suggest removing all usb peripherals out side of the case, shutting down and remove the power supply. Instead of waiting a for 30 minutes I would then press and hold in the power switch for a minute or so. Reboot and connect your USB devices one after the other. Worth a try anyway.
iMac Pro review