I decided to do a clean install on my late 2009 27" iMac. I created a USB boot disk of OS Sierra, formatted the hard drive, booted from USB, installed Sierra. Everything appeared to work fine. However, when it boots Sierra for the first time I get a dodgy screen as below. What is confusing is the mouse cursor appears fine wherever I move it on the screen which I think may rule out a graphic card problem. Any advice please? I've reset the NVRAM on the Mac and this made no difference. The OS Sierra welcome screen looks identical with the same screen issue.