Under certain circumstances, your iMac (21.5-inch or 27-inch, Mid 2010) computer my not start up properly. The iMac (Mid 2010) may shut down or stop responding during the startup sequence. The computer may display a gray screen with an Apple logo, and a  gray progress bar that may complete before the computer shuts down. The hard drive may not be recognized or appear to be unrepairable when you run Disk Utility (located in the Utilities folder) and attempt to verify or repair the volume.
read full article