Tried everything, but still not able to boot my iMac and macbook pro (they BOTH have the exact same issue, at the exact same, after installing macOS Sierra on a clean disk).
– Disk repair
– NVRAM reset
– SMC reset
– Erase disk and re-install Sierra: Installer finished successfully, after reboot the same grey screen
– Erase disk and re-install El Capitan: Installer finished successfully, after reboot the same grey screen
– Safe mode sometimes works, but can’t find relevant log files
– Replaced the HDD with a new one, still same issue