My mid 2011 27 inch iMac screen just goes to a grey screen with five vertical galah pink bars on it.
It fails to complete boot, as far as I can tell, behind the bars you can see the Apple logo and the progress bar, it nearly completes and then it goes to a pale grey screen(without bars) and then self reboots after a while, with the cycle repeating. No log on screen is reached.
This happens whether I use the installed hard disc or an external thunderbolt drive as start up.
This iMac had its video card replaced under the recall years ago, but of course now is well outside any technical support. Any work around anybody can suggest?