Windows 10 has already being installed before doing the following:
in UEFI mode like I've done with a few minutes ago, just for test with the Windows 10 USB Stick.
I had to boot it in Legacy mode, in order for the USB Stick of Ubuntu 16.04 LTS "Xenial Xerus" to run.
GRUB has been installed on ext4, dev/sdb2, sda was the USB Stick.
All done, now in UEFI starts only Windows 10 automatically, and in Legacy mode, no system to start at all.
Now, when I try to access from WIndows the partition where Ubuntu should, I can't access it, and it asks only for formatting it.
Do you consider that if I would do this with a full Lubuntu (not Ubuntu) installation would work?
Any ideas please? Thank you.