In my BIOS I have a section on which I can see all connected SATA devices, and there I can see all my drives. But it doesn't show up as bootable in the boot section.
I realized something when I tried to access my 2TB drive (no OS, just storage) that I can't. I can only access the 2TB drive and the 1TB (with the OS) when I mount it (I also have to "log in" with my password). Does the fact, that I can't boot in it, has something to do with it?