https://askubuntu.com/questions/977170/osx-upgrade-causes-ubuntu-to-magicjicaly-remove-grub-2017-to-high-sierra
dual boot - OSX Upgrade Causes Ubuntu to "magicjicaly remove" Grub?! (2017 to High Sierra) - Ask...
Why does an OSX Upgrade Causes Ubuntu to Call Grub or Crash the Ubuntu Boot Loader? (from a 2017 OS X Sierra 10.12.X to High Sierra) Dual Boot (OSX/EFI Ubuntu...