Certain iMacs sold between October 2009 and July 2011 may have defective hard drives.
All the affected drives are 1 TB models. Apple is replacing them for free. You need not be experiencing any problems to qualify.
Don’t take chances with your data. If yours is one of the affected models, back up your drive right away, and have it replaced.
For more details, and to see if your iMac is one of the affected models, click here.