Lost cluster
Segment of data that is either corrupted or missing, causing the computer to believe that the data is being used when it is not. When getting lost cluster hard disk drive messages it is often a strong indication that the hard drive is failing. If you're running Microsoft Windows and are still able to get into Windows back up all critical data and try running Scandisk on the computer, to mark these clusters as bad. If you are getting lost or missing cluster messages during a format, it is highly recommended you replace the hard disk drive.
Also see: Cluster, Hard disk drive definitions