Ok, we have here an iMac 2008 operating on El Capitan and it can't acheive booting process. After seeing Apple logo and booting bar the screen remains grey and I had to close it with the back button. First thing I've try was to boot on my external clone but same problem. I booted from recovery and did First Aid process. Reboot again and same thing. I went in single mode process and run fsck -y. Reboot again and same problem. Frankly I don't know what to do now. Help would be very appreciated.