Apple has recently launched a hard drive replacement program for Late 2012 27-inch iMacs with 3TB hard drives. This recall affects only a small number of iMacs.
To check if your iMac is eligible use the following link: https://www.apple.com/support/imac-harddrive-3tb/
In order to speed up the replacement process you can fill out a Service Repair Form online here: http://www.gatortectesting2.com/service/mac-repair/