I can't boot my iMac (27", late 2012) and only get to see the white screen. Nothing works, CMD+R, CMD+ALT+Shift+R, Shift, C, CMD+S, ALT … I can hear the startup chime, screen goes white and stays white. I pressed the keys at least for 60 seconds. Neither another NVRAM reset nor a SCM reset brings my Mac back to life.
I tried to use an external HD (bootable, Sierra install on it) but … well ALT key won't bring me the boot selection.
Seems like nothing works. I would like to reinstall my Mac (I've got a TM backup).