White Screen – nothing works after NVRAM reset 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).
Ok, holding N while booting did the trick (Apples NetBoot server).