iMac problem...?

[mullock87]

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.

 

[keysofanxiety]

Hmm, I'd try a reinstall.

Hold CMD+R on startup and select Reinstall macOS. If you want to go nuclear, wipe the disk through Disk Utility beforehand. Wouldn't make a difference either way, though.

 

[havokalien]

Any time I have seen screen artifacts it was a video card issue. Have you tried starting up in safe mode? that disables acceleration and usually and video card issues until reboot. If that works you either need to take out and reflow the card or get a replacement.