Apple has determined that certain Seagate 1TB hard drives used in 21.5-inch and 27-inch iMac systems may fail. These systems were sold between October 2009 and July 2011.

Symptoms can include your iMac running very slowly - much more slowly than normal.  Also read/write errors occurring while copying files, backing up, migrating etc.  It can also cause the iMac to suddenly no longer start up, showing a "no-entry" symbol on the screen instead.  While using your iMac, you may also notice that you see the spinning "beach ball" much more frequently than normal and it stays for much longer than expected.

Apple have accepted that there is an issue with a particular batch of hard drives used and they have extended the warranty on affected iMacs to cover the replacement of the hard drive.  If you think you have been affected, you can check your iMacs eligibility for the hard drive replacement program here:

http://www.apple.com/support/imac-harddrive/