Do you have an iMac purchased between October 2009 and July 2011? Does it have a 1 TB (Terabyte) hard drive in it? If so, it might be part of an Apple recall.
You can locate the serial number of your iMac by clicking on the Apple menu and selecting About This Mac. In the window that appears, click on the numbers that appear below “Mac OS X”. When you click on the numbers, they’ll toggle through displaying the version of Mac OS X, its build number and the Mac’s serial number. Write down the serial number, then go to Apple’s iMac recall page and enter it in the search field to see if your iMac is affected.
[Update February 2017: Mac OS 10.11 and beyond will show your Mac’s serial number on the initial About This Mac page.]