Boot computer into your LL2.8 installation, open a terminal and post back here with output of following commands:
lsblk -af
sudo parted --list
Ubuntu is first and boots first from the grub menu
Is Ubuntu first on grub menu because you installed it after the LL install, or did you change boot order in LL's grub to make Ubuntu boot first? After new install of LL3.4, do you want to keep Ubuntu as first boot choice?