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No need for installer. You can install it from your running High Sierra. Just launch the Install Mojave App from HS and select the 2nd HDD as the destination drive. Make sure to have it formatted to GUID / Extended Journaled prior. On reboot, select the new boot option "Boot from Mojave Install...."

Strange thing... Before, when I was trying all layouts, 13 was not working. Now it works, but I have to check it everything working properly. I will let You know. But for now, thanks a lot for your help