Apple has extended its 1 TB Seagate hard drive replacement program to include all iMacs manufactured between October 2009 and July 2011. The original replacement program only covered 2011 models of the iMac.
Seagate manufactured a batch of 1 TB drives for Apple that are showing early signs of failure. The original issue was with drives used in iMacs built between May 2011 and July 2011. But Apple has determined that the same 1 TB Seagate models were used in iMacs dating back to October of 2009.
If you have an iMac that matches one of the affected serial numbers, Apple will perform a free drive replacement. You can find additional information about the replacement program, as well as check your serial number, by going to iMac 1 TB Seagate Hard Drive Replacement Program.