[ubuntu] Upgraded my MBP to OSX Yosemite which took over boot. Need tool update GRUB

I've got a dual boot set up on my MBP with OSX and Ubuntu. Upgraded to OSX Yosemite this morning, and the OSX OS took over the boot so that GRUB is no longer active. I found out that if I can get into Ubuntu once, then in a terminal window doing "sudo update-grub" should re-establish grub. And there is a bootable usb software Super Grub2 Disk which will boot my into Ubuntu. But I've been trying for hours to make a bootable CD. My other Ubuntu computer (Startup Disk Creator) gives an error when trying to do it. And in OSX the Disk Utility does not recognize the software and will not make the bootable USB. The same is happening with another software by same group, called Rescatux. Unable to make bootable USB.

Is there another tool for getting booted once into Ubuntu? Or any other guidance on how to get GRUB active again so I'll be able to boot into Ubuntu?