I already configured the order of booting, as you said sometimes I get no OS found errors but there a direct method in my machine to boot from usb/optical drive without going bios.
If you install on external, it should be logical to install grub on the external drive (check first…)
I'm planning to re-install everything,
now windows won't boot if I didn't connect my ext drive for the grub menu.
being able to go to windows (with my ext connected) , and if I delete the partition I created on the ext hard drive I guess I get will grub rescue should I delete mbr file manually if so where to find that ?