Unable to boot into Ubuntu 13.04 on Mac
You can't simply install Ubuntu on your internal drive, because Apple's EFI and GRUB (the bootloader) don't get along.

You will also need an EFI bootloader, like rEFInd (a fork of rEFIt):


  rEFIt is a boot menu and maintenance toolkit for EFI-based machines like the Intel Macs. You can use it to boot multiple operating systems easily, including triple-boot setups with Boot Camp. It also provides an easy way to enter and explore the EFI pre-boot environment. Source: The rEFIt Project