The big problem is that with mac os Mojave the original stock gt 120 wont boot. and otherwise i cannot see any boot screen.
Sorry, but I felt like we're going no where here, I might miss understand your goal and/or your problem.
Your goal: to be able to boot between Mojave and Windows 10, without boot screen.
If my understanding of your goal above is correct then:
- 'You do not need boot screen to be able to switch between Mojave and W10"
My setup:
- Mojave in SSD
- W10 in HDD (installed with CSM mode, NOT EFI and NOT using Bootcamp Assistance)
To boot to W10 from Mojave without boot screen (you'll use start-up disk once booted in Mojave):
- Once inside the Mojave, choose W10 (BOOTCAMP) in the start-up disk pane
Referring to the picture I attached, you can see I've got 'BOOTCAMP' aside from the "Mojave".
So if I want to boot to W10, I'll choose 'BOOTCAMP' then restart, then when restarted my MAC will boot into W10.
To Boot to Mojave from W10 (again this is also didn't required a boot screen as we'll choose the startup disk in W10).
- Once inside W10, use the bootcamp start-up tool (similar to the start-up disk pane in OSX) to start/activate the OSX as your boot -> click restart. This required Bootcamp 6.1 please refer to my earlier post for the link on how to install bootcamp 6.1 in W10.
Above are the method to switch between OSX and W10 without boot screen as you'll make the selection once you're inside the OS either W10 or Mojave.
From your first post, I think you're in W10 now and not able to boot to Mojave, follow the link to the guide on how to install bootcamp 6.1 (the guide is to install W10 but you can skip to the steps to install Bootcamp 6.1), once installed then you can use the startup-disk tool from the bootcamp to boot to OSX.
Now your issues:
- Can't see boot screen to choose which OS to boot --> answered: using above method to switch between OS no longer require a boot screen.
- Can't boot to Mojave with GT 120 --> answered: removed the GT-120 as you do not need to see boot-screen any more.
- Can't select OSX from W10 --> answered: install the bootcamp 6.1 as I've sent you a link on how to install bootcamp 6.1 in my earlier reply, in-fact the link also teach you to install W10 using CSM mode.
Unless I miss understood your issue or you've got other issue I missed, you should be good.