I'm attempting to make a USB that contains a full install of Ubuntu (i.e. NOT just a Live CD) that will boot on PCs or Macs. Making it bootable on PCs is fine, and I've successfully achieved that.

However, making a Mac-bootable USB is proving difficult. I followed (with some modifications, due to what I'm trying to do) the steps shown here:

http://ubuntuforums.org/showthread.php?t=1065670

Following that, I have a USB that can boot on a PC as well as a Mac laptop from a couple of years ago (although I'm not entirely sure what model it is).

However, upon trying the same USB on a newer Macbook Pro (from late 2011) the GRUB menu was not shown. After selecting rEFIt from the MAC boot menu and selecting Linux, the error about not being able to find any bootable devices is shown and Ubuntu is not accessible.

Does anyone know what might have changed with the newer Mac that does not allow one to access GRUB?