the two posts saved my day

搞了6個小時, 歷經無數次的reinstall, 試了一堆insanelymac的老式 無用方法, 結果仍是ubuntu社群給的答案最好. 

use 'multiboot' in grub2 to chainload chameleon  and OSX

http://ubuntuforums.org/showthread.php?t=1496322

how to reinstall grub2 if your MBR is damaged by other OSes.

http://www.dedoimedo.com/computers/grub-2.html#mozTocId842078