Apple announced a replacement program for some 27-inch, 3TB hard drive models yesterday, June 16, 2015. The reason for this recall is due to an issue that could see these models fail under certain conditions, and so you have been asked to fill in a form in order to have your iMac replaced.
If your 27-inch, 3TB hard drive iMac was built from December 2012 and September 2013, and so was the late 2012 update, then your model will be affected. However, Apple has not offered details of a specific product number
With this being the case, you have been asked to provide your product number, if you left a valid email address when you purchased your iMac, then Apple will then let you know if it is included in this program.
However, do not worry too much, as Apple has already started to contact affected owners, it’s just the replacement program page has been put in place to help speed the process up.
The only thing to be replaced in this program is the 3TB hard drive itself, so you might wish to start backing up your files now.
However, Apple may do this for you, so you might want to get rid of any files you do not wish anyone to see.
You will then have three options: Set up an appointment with a Genius, use an Apple Authorized Service Provider, or contact your local Apple Technical Support.