My current config:
1) SSD with Debian Stretch, Legacy mode, Grub2 edited to list Windows 10 aswell.
2) HDD with Windows 10, Legacy mode, with own bootloader.
Problem is, i go to BIOS to set SSD as first boot priority, but it's not there..., to be able to boot SSD i have to press F11 while booting my PC and select it on the list.
This is a weird problem and i'm not sure if it's Debian related or just stupid BIOS.