I just installed ubuntu 18.04.4 in my pc. I had already windows 10 and it is dualboot with linux now. I figured out that I have to set the priority #1 to the drive where I installed linux in order to display the grub-bootloader. But when I go to the boot priority to make the arranges, my hdd is not showing up to set it as the first. The hdd is recognised with the sata cable and it's fine. When I try to boot hdd without changing the priority, by just clicking the hdd on bios from boot menu just for one time, everything runs okay. I installed linux to a partition of the hdd. Windows 10 are on SSD. The root file I selected during installation is ext4 journaling file system. Any ideas on that? Thanks in advance!
Obviously the partition on your HDD is bootable. Otherwise you would not be able to boot from it by manually selecting it in the BIOS.
Check your BIOS and change the boot order, so that GRUB is the first partition to boot from. There you should configure all the other OS you want to boot to (like your Ubuntu and Windows). Trick: Ubuntu usually sets already the boot record in GRUB for windows partitions, which have been found during the installation. So you only need to change the boot order in BIOS, to make it work.