http://www.apple.com/support/imac-harddrive/Not all 21.5-inch and 27-inch iMacs with factory installed 1TB Hard disk drives made in this time frame are affected. I know this first hand as my 27" iMac was not.
To see if your iMac is or is not affected go to that link and enter your serial number. You can find the serial number by clicking on the Apple icon in the upper left corner of the main tool bar, then clicking on "About This Mac" and then clicking on the "More Info" button.