Apple has disclosed that a “very small number of 3TB drives used in 27-inch iMac system may fail under certain conditions”. These 27″ iMacs were manufactured between December 2012 and September 2013.

In response to this, Apple has created a program to replace affected drives free of charge, regardless of warranty status. To see if a 27″ iMac qualifies for this replacement program, enter it’s serial number here: https://www.apple.com/support/imac-harddrive-3tb/