Thank you Jesse. I read the listing of Windows 10 compatible computers wrong. Live and learn. 
XPS8500 with fully functioning Windows 10 installation. Tried to install W10 Upgrade. now screen blanks/rewrites in loop. Broke out of loop but can't restore to prev version. Init ok, Backup data ok, halts on HD preparation with 0x 40011002000001005 error.
screen blanks/rewrites in loop after install Win 10 upgrade to XPS8500