Thank you for the link.
Today i flashed the ati radeon hd4850m 512mb mxm card with the apple ati radeon 4850 bios from techpowerup.
But the internal screen does not show the white efi start screen on booting up the imac.
Also there is no efi white boot screen on external monitor connected via display port.
Furthermore whan os-x is booted there is a normal Screen with internal and external display.
I think there is something wrong with the bios or the efi partition is not there
Can anybody with an imac 27 and ati radeon hd4850m installed be so kind and make a bios dump of his card with atiflash -s 0 hd4850.rom and send it via email.
I will donate him with 10€ via paypal.