Today Apple announced a replacement program for the 3TB hard drive in 27-inch iMacs sold between December 2012 and September 2013. The company has determined that "a very small number" of the all-in-one machines were manufactured with a drive that "may fail under certain conditions." Since late 2012, iMacs have offered the huge 3TB storage option as part of Apple's Fusion Drive, which pairs a smaller, faster flash-based drive with a capacious HDD. The 3TB hard drive has never shipped in any base or entry-level iMac configurations, but has been available for professionals and consumers who need more built-in space for photos, video files, music, and other media.
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Apple says it is the process of contacting customers who purchased an affected machine, and you can also check to see if your Mac is included in the replacement program by entering its serial number over at Apple's website. The company will swap the 3TB drive for free, regardless of whether your iMac is still in warranty. You can take the iMac to the Genius Bar at an Apple Store (appointment required) or any authorized Apple service technician can also do the job. But Apple does note you'll need to reinstall everything once it's done, so if you've got one of the machines at risk for hard drive failure, be sure to back up everything through Time Machine before going through with the replacement.