08-08-2012 09:13 PM
All USB ports in all the 2009 iMacs were USB 2.0. While you can use a USB 3.0 device, it will only operate at USB 2.0 speeds. You will not receive any benefit for using a USB 3.0 device over a USB 2.0 device, but it should still work.
Hope that helps!
Senior ConnectPro Computer Technician @ #615: Orangeville, Ontario