iMac (Mid 2011): Unresponsive white screen on startup or vertical lines on screen
In certain circumstances, your iMac (Mid 2011) may become unresponsive on startup, with a white or gray background on the display. You may also notice vertical lines on the display while using your iMac.
Install the iMac Graphics Firmware Update 2.0 if your computer exhibits these issues. To install the update, choose Software Update from the Apple () menu and follow the onscreen instructions.
Important: Do not interrupt the power to the computer while the update is installing. After the update is finished installing, you will need to restart your iMac.