I have successfully installed TrueOS from TrueOS-Desktop-18.03-x64-USB.img on my second HDD. Strange enough, it installed a BSD boot loader that can boot a previously installed FreeBSD and GRUB from the first HDD, but not TrueOS itself.
The boot menu shows the following entries:
F1 FreeBSD (should be TrueOS)
F5 First HDD
When pressing F2, it boots FreeBSD. Pressing F1 starts GRUB from the first HDD. Pressing F5 or F6 also starts GRUB from the first HDD.
So what kind of magic do I have to apply to start TrueOS?