I have an existing system with Windows 7 64 bit installed on one HDD. System is BIOS ROM. There is a second HDD, no OS just files. Both are formatted with Windows partitions.
I want to add Ubuntu 16.04 to make a dual-boot system. I don't want to change the existing Windows installation.
Using the installer on the Live CD, after adjusting the partitions on the second HDD, can I install Ubuntu on the second HDD?
Will this keep the existing Win7 installation unchanged?
As for selecting which OS to boot, which HDD will the MBR be on? And will it still operate correctly to select which OS on startup?
Or would it be better to select the OS by setting the HDD boot order in ROM?
Any other considerations or flaws with this plan? And part that couldn't be done using the Live CD installer?
You may install Ubuntu on any volume in your system, as long as you install GRUB to the drive the system boots from. As noted in comments, one way to avoid making any changes to the Windows drive (allowing easiest return to single-boot Windows-only operation) is to change the boot order in your BIOS settings to make the empty drive primary, then install Ubuntu on the empty drive. During installation, Ubuntu will set up GRUB to boot Windows, so you can still use Windows, but if you want to return to "one OS" operation, just change the boot order back (so the Windows drive is primary).
I had that setup for several years on my desktop machine (the one I'm answering from), so can confirm it works.
If you're able to you could keep the Windows HDD unplugged during the Ubuntu installation. Then each drive will have its own MBR and bootloader which you could switch using the bios.
This method makes it easier to uninstall either operating system later as you won't have to worry about fixing any broken bootloaders.