I started from scratch on the boot disk using my other MacBook Pro and a YouTube video to help me with the terminal coding. After recreating the boot disk (it took awhile to churn through 6 Gb) I was able to boot from disk and reinstall High Sierra on the ailing MBP. All went well last night as I reloaded software I needed, including Microsoft Office stuff, Dropbox, and Chrome. All was fine, slow, but fine. Today. Won’t boot all the way, won’t boot in safe mode, and there are pastel horizontal lines on the screen. The machine is ten years old and I have kids at home doing schooling (Honestly, I think this is what wore it out!) who need a computer, so my ultimate solution was to buy a new MB Air, on sale for $800 at BestBuy. I briefly considered taking the old MBP in to a shop, but after ten years I think it’s time to let it go. Thanks for all the help on this thread!