Well, mystery solved - response headers were being set, just not obvious with IIS 6, have to track them down via DEBUG/Watch with code that can enum the Custom Header array ((System.Web.HttpResponseHeader)((new System.Collections.ArrayList.ArrayListDebugView(Response._customHeaders)).Items[0])).Name "Accept-Ranges" ((System.Web.HttpResponseHeader)((new System.Collections.ArrayList.ArrayListDebugView(Response._customHeaders)).Items[0])).Value "bytes" Able to verify Header set, and respond to resume download 'Range' Thanks all
Tom