@michaelmarcus My iMac is late 2012. This is all from memory and wasn’t necessary on my machine as it is distinctly older but try booting into Recovery mode (Apple-r) and look for security settings. You may need to switch on the ability to boot to external media which could be off by default
@SGgrc Spinrite 6.1 RC6 running directly on my old iMac and booting off usb. I couldn’t do this before. Now it’s easy. Thanks for Spinrite and Security Now…