Ursprünglicher Beitrag von: nfiertel ,
Try to boot using option key on an priginal apple,keyboard. If,there,os a working graphics card you will see the choices,for any startup disk. I,for example,have a newer card in a 2008 Mac Pro which glitches on startup as those card,is only sort of supported,but using the option key allows,it to,work,fine once I choose the appropriate startup disk. It,should not be this way of course but this works.for me. It does sound as if your graphics card is not installed or os defective,or cables are inappropriate. There are two outputs so try the other alternative outputs.