Installing macOS on a new HDD from USB drive stuck on grey screen with Stop sign
My old MacBook Pro is a mid 2009 model which is not supported by macOS Sierra.  - Only MBPs Mid 2010 and later are supported.

I created an bootable USB disk from El Capitan (which I re-downloaded from "My Purchases" in AppStore) and that is booting up on my old MBP. Installing it now.