I have installed Mojave, on my unsupport iMac 9.1. But now I want to downgrade again to the best supported system. The problem is that I don't have El Capitan under my Mac AppStore downloads because it was downloaded through another Apple ID that I don't have access to and I can't download it directly from the AppStore since Mojave doesn't allow me that. Otherwise I had El Capitan on a USB key, but got it overwritten with Mojave. Beautiful. Does anyone have good advice on what to do?