i am trying to help a friend clean install his 2008 imac with el captian. He had upgraded to el capitan a while ago, but for a while now he could only boot in safe mode. As I have performed a number of fresh installs on other machines, so I thought "no problem". I created a bootable usb with el captian on. Booted up with it, wiped the hard disk, then ran the installation. Seemed to run ok, but when complete it just hangs on a black screen - can't see a mouse cursor. Tried twice, same result. Any ideas? Only other thing i noticed was when i booted up with the USB, the screen had grey circles on it (in the background, large, like a ripple in water), wondered if that signified anything?