What I don't get is why the USB / ISO methods reject my Windows 7 product key. This may make no difference to my constant failure to upgrade to Windows 10, but is an irritation. I checked the product number by retyping it into Windows Activation, so I know it is right.
Incidentally, if you lose your Windows Product Key you can easily retrieve it using this clever app.
http://www.nirsoft.net/utils/product_cd_key_viewer.html
Just scroll to the bottom of the page to find the download links.