…from: https://www.apple.com/support/imac-harddrive-3tb/
iMac (27-inch) 3TB Hard Drive Replacement Program
Apple has determined that a very small number of 3TB hard drives used in 27-inch iMac systems, may fail under certain conditions. These systems were sold between December 2012 and September 2013.
Apple or an Apple Authorized Service Provider (AASP) will replace affected hard drives free of charge.
Apple is contacting affected iMac owners who provided a valid email address during the product registration process to let them know about this program. If you have not been contacted, but think you have a 3TB hard drive, you can enter your serial number below to see if it's part of this program.
Finding your iMac serial number is easy. See how.https://support.apple.com/en-ca/HT201626 The serial number you entered is not eligible under this program because either: - It's not one of the serial numbers in the affected range OR - Our records show that your hard drive has already been replaced. If you paid for hard drive replacement, check with Apple to see if you are eligible for a refund.
Replacement Process
Apple recommends replacing affected hard drives as soon as possible. Please choose one of the options below:
* Apple Retail Store - Set up an appointment with a Genius. * Apple Authorized Service Provider - Find one herehttps://locate.apple.com/ca/en/. * Apple Technical Support - Contact us for local service options.
Before you go in for service, Apple strongly recommends backing up your data. Learn more about backup optionshttps://www.apple.com/ca/support/backup/.
Additional Information
You will need to re-install your operating system, other applications and any backed up data after your hard drive is replaced. Please refer to About OS X Recoveryhttps://support.apple.com/en-ca/HT201314 for instructions.
If you believe you have paid for a repair or replacement due to this issue, contact Apple regarding a refund.
This worldwide Apple program does not extend the standard warranty coverage of the iMac.
The program covers affected iMac models until December 19, 2015 or three years from its original date of sale, whichever provides longer coverage for you.
Information as of 2015-06-19