If you purchased a new 2011 iMac with a 1TB Seagate hard drive between May 2011 and July 2011, Apple has reported that the 1TB Seagate drives within the systems might fail, and has now started a replacement program to help you change the drive before any data loss is experienced.
Apple has started contacting customers who have been affected by the problem via email. So don’t wait until your hard drives fails, book your iMac in to Apple Store to have your Seagate 1TB drive replaced for free.
Apple has determined that a very small number of Seagate 1TB hard drives used in 21.5-inch and 27-inch iMac systems, may fail under certain conditions. These systems were sold between May 2011 and July 2011. Apple or an Apple Authorized Service Provider (AASP) will replace affected hard drives free of charge.
You can enter your iMac serial number into the Apple site here to check if your system is affected.