Ran Produkey and "Enchanted Keyfinder" on a somewhat dead old laptop hard dive. Computer had been upgraded from Windows 7 to 10 Pro using the upgrade wizard about 2 months ago.
Both product key finder programs list the same product key identified as "Windows 10 Pro". I installed a new hard disk, ran the DVD installer and when I input that Windows 10 product key, it says, "This product key didn't work". Checked for typos multiple times.