Thanks for this explanation. I re-installed 16.04 from scratch, erasing the disk. Not without difficulties, though, as I got an error message about not being able "to mount a file system with type vfat in SCSI1" (fixed that by booting from USB into a live system and writing a new GPT partition table before re-attempting installation onto the now empty disk with empty partition table).
The new installation was done in UEFI mode. A note about the warning message that you referred to, i.e.: "This machine's firmware has started the installer in UEFI mode but it looks like there may be existing operating systems already installed using "BIOS compatibility mode". If you continue to install Debian in UEFI mode, it might be difficult to reboot the machine into any BIOS- mode operating systems later. If you wish to install in UEFI mode and don't care about keeping the ability to boot one of the existing systems, you have the option to force that here. If you wish to keep the option to boot an existing operating system, you should choose NOT to force UEFI installation here." On re-reading this message in light of what you explained it now makes sense. At the moment, however, I didn't realise that "one of the existing systems" actually included the one I was trying to repair (which was the only system installed). Somehow I thought the message referred to Windows or other systems. The dialogue box then provides two options: "Go Back " and "Continue in UEFI mode". What I was missing to see was an option to "Continue in BIOS mode". Am I right in guessing that that needs to be selected in the BIOS before rebooting the machine? -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1575092 Title: Reinstall of Ubuntu 16.04 over failed upgrade of Ubuntu (15.10 -> 16.04) results in grub installation failure To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/grub-installer/+bug/1575092/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
