This is a working, and easy, solution: (thanks Patiek)
Since windows was booting fine, original MBR on sda ok.
Unplug sda from sata port #1, keep sdb plugged into sata2 port #2.
Rerun whole install.
Fedora creates a new MBR for the secondary drive.
Continue with installation untill Fedora is working properly.
Shut down. Plug in sda into sata2 port #1.
Switch between Fedora and sda (WinXP for me) via BIOS mobo boot order.
Procedure was recommend since there were other unknown conflicts in my disaster of an installation attempt