Own a 27-inch iMac with 3TB Hard Disk Drive, you may want to check your unit is a part of the replacement program. Apple today announced a replacement program for the 3TB hard disk drive in 27-inch iMacs old between December 2012 and September 2013.
There are a very small number of the iMac were manufactured with a drive that may failed under certain conditions.
You may check whether your unit are affected by entering serial number over at Apple’s website. If you are the unlucky one, Apple will swap the 3TB drive for free, regardless of whether your iMac is still in warranty. Make sure you back up everything through Time Machine before going through with the replacement.