[CentOS] Finally.... CentOS on iMac core 2

I've been trying several combinations of OSX, CentOS to try and get CentOS
installed on an old iMac. I finally first installed OS X, then installed
CentOS in the open space after OS X. With refit installed and selecting
CentOS, it starts booting but get a screen that a boot device can't e
found. So I then install Xubuntu with the option to replace OS X.  After
Xbuntu is installed and then do a reboot the grub screen comes up and I can
now select CentOS and it will boot.

Can someone explain why this is?  I can't just install CentOS on the whole
disk, as I get the blinking mac disk with question mark.

Thanks,

-wes