I'm unable to correct set my dual boot. windows 8 and ubuntu 18.04 on a samsung EVO 860 SSD

[romanial]

I'm triyng to setup a dual boot from a Samsung EVO 860 SSD.
sda1: Windows 8
sda2: /
sdb5: /home
Neither, placing the mbr on sda nor sdb, let me to correctly install the Grub.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.6 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CasperVersion: 1.394
Date: Wed Oct 3 11:16:27 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu-mate.seed boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu-MATE 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
ProcEnviron:
 LANGUAGE=ca_ES.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=ca_ES.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)