dual boot - Installing Ubuntu on a separate hard drive alongside Windows 10

I have some trouble with installing Ubuntu on my Windows 10 laptop. My laptop has two drives. one SSD (which Windows is installed on) and one HDD (on which I store my data). I want to install Ubuntu 16.04 on my HDD. I tried to install it, but now I can't boot Ubuntu or select between Ubuntu and Windows, so I wanted to ask what I think may have gone wrong.

1. My BIOS type is in legacy mode and during installation I get the following warning:

   
   
   

   
   

   This machine's firmware has started the installer in UEFI mode but it looks like there may be existing operating systems already installed using "BIOS compatibility mode".
   If you continue to install Debian in UEFI mode, it might be difficult
   to reboot the machine into any BIOS-mode operating systems later.

   
   
   

   
   
   

   I decided to select "Go back" instead of "Continue in UEFI mode" to avoid messing with Windows 10. And it proceeds normally to the next steps of installation. Is it OK?




2. In 2 more steps I got this message.

   
   
   

   

   
   
   

   I wonder why installation doesn't detect that my other OS is Windows. Anyway I continued with Something else.




3. In the next step I go to the partitioning part. I already formatted one of my hard disks in order to install Ubuntu on it which you can see in this picture.

   
   
   
   

   

   
   
   

   It seems that I have to shrink this volume into two or three (not sure) on assign each one of them something like (/, /home, /root) which I don't understand what are they. There is also a mysterious panel for "Device for boot loader installation". I proceeded with making the whole drive as (/). but still there is a mysterious panel for "device for boot loader installation". I select it to be my (/) drive. I proceeded and installations seems to be finished but now, I cannot boot Ubuntu. (my secure boot is disabled)

Have I done anything wrong or missed anything?

I used the link shared by dear mAnN for partitioning my drive. I tried to install Ubuntu on a partition which starts somewhere in the middle of my drive. The installations ended normally and Ubuntu booted just fine. But after rebooting it happened to lose the path of Ubuntu's bootloader (grub). To be precise, Here is what I faced with: if my boot priority was set to be my HDD (Ubuntu's host), after every single reboot I would see the grub rescue message.

I could boot into Ubuntu by redefining the path with the following commands from secure grub:

set prefix=(hd0,msdos1)/boot/grub
insmod normal
normal

but that was for sure a painful procedure. Moreover, I wasn't able to boot into Windows either unless I changed the boot priority.

I repartitioned my whole HDD (as the basic drive) such that my first partition is the one which Ubuntu is installed on (this partition shrank into root ("/") and home ("/home") during installation later) and installed Ubuntu. This time surprisingly everything worked out! Now everytime after turning on the machine, I see a menu which I can choose which OS I want to boot from. Note that my main boot priority is set to be my HDD which contains Ubuntu, not Windows.

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PS:I'm not sure if unallocating my HDD, repartitioning and installing on the first partition was the real reason that fixed the grub problem. Maybe the problem was much easier and just something had gone wrong during my first installation.