Apple has started a replacement program for a “very small number” of Seagate 1 TB hard drives that may fail in certain conditions. These drives came in 21.5-inch and 27-inch iMacs sold between May 2011 and July 2011. Although Apple is contacting iMac purchasers who provided a valid email address during the product registration process, you can also enter your iMac’s serial number on this page to see if it’s included in the replacement program. As always, back up your data regularly, and be certain to run a backup right before you take your Mac in for the replacement so you can restore onto the new drive.