Dear people,
Current op-sys on my house desktop is Debian 8.6 (Jessie) which boots
from a dual personality UEFI/Legacy BIOS set to Legacy mode. I want to
install Debian 9.3 on a separate 'primary' hardisk. The Debian 8.6 will
be relegated to s separate secondary harddisk. I still want to
optionally boot Debian 8.6 via grub. My question concerns the
installation of Debian 9 and the setup of the grub boot loader.

1. I understand that Debian 9 can be installed under UEFI and will
seek out the presence of a UEFI BIOS; but can I force the
installation in Legacy mode anyway? If so, how? (Dinosaurs are too
old for unwelcome surprises)
2. My version of Debian 8.6 probably won't boot up via a UEFI BIOS.
If one is forced to use UEFI how should I arrange grub for a dual
boot?
Or is it a lost cause?

Thanks in advance to anyone who can help.
Dynosaw

Generally one is forced to use uefi on systems that won't allow legacy/csm. Or on systems that don't provide hardware in legacy.

If your system allows legacy then you can easily install Debian 9 that way. I can't remember if older versions support uefi but I think they did in 8. See Debian web site.
I suspect you'd have to create uefi boot/install media.

Never a quick way to boot from one to the other. Always have to go to bios and switch back and forth. Some bio's require two switches to get video and legacy back from ueif.

If you have Debian 8.6 installed on a separate hard drive with it's Grub boot code in the MBR of that drive, you can select that drive on boot regardless of what is on the second drive. On my system with several UEFI Linux installs on the internal drive, I can boot another Linux system on an external drive of an MBR install on that drive from the Grub menu of one of the internal installs with Grub. If you are unable to install in Legacy mode, run grub-mkconfig (or update-grub if Debian uses it?) from the new install once finished and re-booted.

You get the the uefi during bootup/installing it. The installer is universal during boot for my machines I had to use boot option key F12 and choose the EFI (Installer USB) boot option to boot from though even this is not enough to end up with it installed that way if you do not choose manual partitioning of the disk in the machine and create an efi reserved partition for the proper place to put the boot files you end up with legacy mode installed at the end of the process. To get to manually partition you need to have chosen on the expert modes at the start of the install process I think this part is true been so long since I have done any other type not sure if they offer the manual now. This was with version 8 to answer the OPs 1. set the BIOS to EFI only during install choose the installer as an efi boot option in that menu for your board then make sure to have created and assigned for efi booting an efi partition in the manual partitioning option. For number 2. I am thinking having assigned the disk containing the new 9 install as the primary drive to boot from it grub booting in efi mode will just boot the old 8 install after all it is not like they are any different versions of programs one compiled for efi ones not that are installed. The efi booting is dependant on only one that is such a beast the grub-efi package installed into the efi partition on the disk with the BIOS set to boot EFI mode. You get that installed when you have created assigned the EFI reserved boot partition during install.

Took to long yancek seems to have beat me to some of it as I see in a preview.

Dear People,
Thank you for your help, which is much appreciated.
My delay in replying is due to intervening social
obligations which kept me away from home.
I can now say that I have sucessfully installed
Debian 9 (Stretch) on sda under the "Legacy" feature
of the dual personality BIOS and update-grub found
the previous op-sys Debian 8 (Jessie) which had been
relegated to sdb. It all went smoothly except for one
minor point, and I will set this down here, in case
others may find it useful.

The mother board of my stand-alone desk-top PC is an
Asus AX88M-A (2016 vintage). On boot up form a Debian
live DVD with the BIOS in legacy mode the monitor showed
a warning message:
"System found unauthorised changes on the firmaware,
operating system, or UEFI drivers. Press [N] to run
next boot."
This message seemed to be superfluous in this case; I
pressed the N-key and the basic installation went ahead
without any further problems.

I can consider this action closed.
Again thanks to all who replied for your help and advice.

Dynosaw.