Dual Boot: Can't change my boot mode. Why?

My Dell XPS 8500 has two hard drives. Windows 10 64 bits which I have upgraded from Windows 8.1 is installed on hard drive C. I want to install a fresh new copy (not a factory image) of Windows 7 on the second hard drive. I want to install Windows 7 through a bootable usb pen drive. I spent hours trying to boot my computer on the bootable usb drive but to no avail.

I went to the bios to change the boot mode.

Before trying to install Windows 7, my boot mode was:

1st Boot Device:                UEFI: Windows Boot Manager

2nd Boot Device:               USB: Storage Device

3rd Boot Device:                Internal ODD Devices

4th Boot Device:                USB Floppy Device

5th Boot Device:                Outboard Nic Device

and I placed the ‘USB: Storage Device’ in the 1st Boot Device, expecting that my pc will boot through the USB: Storage Device so that I will be able to install Windows 7 on the second hard drive. But it didn’t. It booted on Windows 10 again as if I didn’t do any changes to the boot mode.

On my old pc I used to change my boot mode and I was able to install a new operating system through a DVD but on this computer, when I change the boot mode nothing happens.

What should I do to install ‘Windows 7’ through my bootable usb pen drive on Drive D?

Last Question: Why in my boot mode there is not my dvd drive listed? What can I do so that it will appear in the list?

Please help. I want a dual boot on my pc but I’m stuck. I have searched on other forums and even saw videos on youtube. I have followed or at least tried to follow the instructions on how to change the boot mode but I wasn’t able.