Apple has released a firmware update for the mid-2010 iMac systems (both 21.5- and 27-inch models) that addresses a problem where the systems were not booting properly. The update is a small 767KB download that should be available through Software Update for relevant systems, but can also be downloaded directly from the update's Web page.
The update will require you to restart your computer, and especially since this one affects the storage medium on your system, be sure to fully back up your hard drive before installing. Download and run the installer file, and follow the onscreen instructions to apply the update. Be sure to not interrupt the process of this update, as faulty firmware upgrades can render hardware useless.
If you have a 2010 iMac and have experienced problems when booting your system, be sure to apply this update. There is no information on whether this firmware update is specifically for the hard drive or for the onboard drive controllers, and therefore it is uncertain whether this update applies to people who have replaced their internal drives with third-party hard drives.