Getting Black screen after installing 12.04 on new Mac Pro

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http://askubuntu.com – I installed Ubuntu 12.04 on my new Mac Pro. I had some problems because bootcamp did not allow me to partition the hd without a Windows cd. I inserted a Windows cd and did the partition and then I stopped the installation. I completed the installation of Ubuntu that now works. The problem is that Grub can't start Mac OS. It sees the Mac OS X in the menu but if I try to start it I have a black screen or sometime just the boot manager. Has anyone else experienced this problem? (HowTos)