The Ubiquity installer defaults to installing Grub into the MBR of sda. I have been caught out by this. Also keep in mind that the last version of Linux to be installed will overwrite any existing MBR.
I have two hard disks with multiple versions of Ubuntu so I frequently have to put my preferred Ubuntu back in charge of the MBR and the Grub boot menu. I do this by loading into my preferred Ubuntu and running
sudo grub-install /dev/sda
Depending upon which hard disk has boot priority at the time and which disk the Ubuntu I am running is loaded on. And when I put in another install of Ubuntu I make sure at the partitioning screen that Grub is going to be installed into the MBR of the hard disk that I want it to.
sudo grub-install /dev/sdb
I also found out recently that Windows 8 will put its boot loader into the MBR of every hard disk. This is worse as the Microsoft Boot loader will never offer an option to boot into Linux.