If you own a 21.5 or 27-inch iMac that was produced between October 2009 and July 2011, your Mac may be eligible for Apple’s Seagate Hard Drive Replacement Program. According to this official notice, Apple or an Apple Authorized Service Provider will replace affected Seagate hard drives, free of charge.
You can quickly find out if your iMac is eligible by submitting its serial number, which can be found by clicking on the Apple icon in the top-left corner of the desktop. Click on About This Mac, and then click twice where it says Version…. Your iMac’s serial number will be revealed. The number is also etched onto the bottom of the foot of the iMac (see screen shot below).
Originally, the replacement program covers Seagate hard drives in iMacs produced between May and July of 20011. But after more customer feedback, Apple has extended the replacement program, and now covers affected iMacs for three years after the retail sale of the iMac or until April 12, 2013. Apple says this worldwide Apple program does not extend the standard warranty coverage of the iMac.
After the replacement you will of course need to reinstall the operating system, so be prepared to back up your entire system using the built-in Time Machine, on an external drive, before the replacement is done.
Apple says it will continue to evaluate service data and provide updates to this program as needed.