I just try to install 18,04 LTS over 17,10. I could'nt install it by normal update because 17,10 was buggy, so I use an ISO file. The system crash saying something like "can't install Grub2 in /target/ so the system will exit.

[Pierre Bouchard]

I have 2 disk for dual boot, one for Windows and one for Ubuntu. On 17,10, Grub was installed on the Ubuntu disk.
As 17,10 could not update the files because the kernel package was corrupted by a bug, I try to reinstall Ubuntu with 18,04 using the option: Install 18,04 over 17,10 and erase 17,10.
When I will leave this I will loose definitly Ubuntu and I will have to return to Windows after 10 years of Ubuntu use. That's "THE" real big problem.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CasperVersion: 1.394
Date: Mon Apr 30 16:08:33 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)