Good evening to you, Mike (as Wizard sleeps down under). It is about 10:00am here near Chicago. We are quite spread out across the globe!
You can get your Ubuntu 20.04 USB flash drive ready, but don't rush to install it. There may be more kinks to work out as we go, and the time zone differences will make many delays. Chris may prefer that you start a new thread for this change in direction as it might be easier for future readers to find a thread that is specific to installing Linux to a 2nd hard drive.
Take note of his remarks above and check your UEFI/BIOS settings to be sure that:
1. No RAID configuration is enabled
2. SATA controller setting is AHCI
3. (My addition) Check if you have a "TPM" or other type of "Trusted Platform" or other security modules enabled. Not necessary to disable it at this time, and probably will not.
I will describe what in my mind would be "ideal"... but I know that you may not go this far. That's okay. I might not go that far either. If your 4TB backup drive is internal, we can probably work around it (unless Chris thinks otherwise)... but if it is USB, then definitely unplug it to minimize confusion.
In an ideal world (and with backup drive removed)....
#1. I would start with only one SSD installed (and plugged into Drive 0 on your motherboard, if you can determine that). I would make UEFI/BIOS settings so that UEFI is enabled (Legacy disabled, also sometimes called "BIOS Mode" or "CSM" - Compatibility Support Module) and I would enable Secure Boot. I would boot the Ubuntu USB flash drive and use GParted to make this first SSD a "GPT" drive (GUID Partition Table) but I would not format it with GParted, just leaving it as unallocated space. GParted should identify this drive as /dev/sda. Installing Windows 10 now will format and partition the SSD as it should be for UEFI. Let Windows get any needed updates.
GPT and UEFI are the modern methods. MBR (msdos) and BIOS are very, very old (like Chris and me
). Obviously Windows 10 would prefer modern methods. Some Linux distros are falling behind, but Ubuntu is also very modern and should be able to use UEFI and Secure Boot. I'd only disable them if absolutely necessary. Your UEFI/BIOS settings (UEFI and Secure Boot both enabled) will apply to both hard drives.
Windows will probably create at least 3 partitions, and one or two small unallocated space areas. One of the partitions will be FAT32 and will hold your UEFI bootloader. With Windows 10 running properly, I would shut it down and remove (or disconnect) that drive.
#2. Then, install the 2nd SSD. Drive 0 on the motherboard is still okay, so you can use the same cable for now, or you can use the Drive 1 motherboard cable, either one. Booting up the Ubuntu 20.04 flash drive, I would run GParted to make sure that this drive is also set to GPT, and change it if necessary. Again, leave only unallocated space on the whole drive. Start the Ubuntu installation from the desktop icon. At the partitioning step, the best choice is to "use entire disk." With there being only one SSD installed, there is no confusion about where to put the GRUB bootloader. Probably all the default choices will work fine during the install. I never recommend new users to choose "LVM partitions" or to use any encryption that is offered. They are certainly useful, just not good for new users. With UEFI enabled, you will get two partitions: a FAT32 boot partition and an EXT4 Linux partition that contains everything it needs (including a swapfile that you don't see). When finished, reboot the new system and let it update.
#3. Add the Windows drive back in with the Drive 0 cable, if you can tell. If there is a BIOS boot hardware preference, it will be Drive 0, or it may recognize Windows and prefer that regardless. But as you will know, both of your drives will be very independently setup, and you will need your BIOS Boot Menu to select Ubuntu. This time, with any luck, you can run
sudo os-prober
and
sudo update-grub
and Windows will be recognized. Then you can make this drive first in your BIOS boot order and use GRUB will boot every time and allow you to choose between Ubuntu (which will be default) and Windows.
Now, in the real world, I doubt that you want to reinstall Windows. But just reading all of my description will give you food for thought in the future. And you can probably see all of my reasoning for these steps. But, remember.... stuff still goes wrong. Different computers, different UEFI, different SSD drives, or for whatever reason. Maybe you will need Secure Boot disabled, or other changes, to finally find success.
With that long summary, I'll pause so that maybe you can read it before your bedtime, and we can hope for Wizard's input on the next steps. You can try to enable UEFI in your BIOS settings to see if that will allow
sudo os-prober
and
sudo update-grub
to work. It still might offer you a bandaid solution while we look at your other options more carefully.