Hackintosh startup disk not detected?

Ok, so i recently installed apple on an external HDD (with a PC - hence hackintosh) i every time i reboot the 2nd time after install it fails to boot, so i have done software updates and have looked around, i went to system preferences and startup disk, and it doesn't seem to have detected the mac startup disk which i boot from (i.e. the external HDD). How can i link it to my ext. HDD so i can boot up?