Dual boot with Ubuntu and Windows on separate disk

If you want to be completely sure your Ubuntu installation won't do any harm to your Windows installation, you can unplug the Windows disks out of your computer and strat the installation process afterwards. You'll only see one available disk when you're installing Ubuntu. After the installation is done, you can plug the Windows disks back.

This way, GRUB will only detect your Ubuntu installation, but you'll be able to boot from both Windows and Ubuntu when selecting the boot disk in your UEFI/BIOS.

Installing Ubuntu on Different drive than Windows 10

GRUB will only boot Windows and Linux if they are installed in the same Legacy/UEFI mode

• Determine Windows 10 boot mode. Type System Information in Windows Start panel. Under "BIOS Mode" it will say if Windows is Legacy or UEFI.

• Boot a Ubuntu 20.04 Live USB in the same mode as Windows boot mode. A USB made with Rufus MBR partition scheme option will boot either way. If Windows is UEFI use Rufus GPT scheme.

• Install Ubuntu to it's own SSD using the "Something else" option.

• if asked, create a 500MB EFI system partition on the Ubuntu SSD.

• Select the Ubuntu SSD for bootloader installation.

• Continue installation as directed.

• When complete, set the Ubuntu SSD as first hard drive, run sudo update-grub to ensure all bootable drives are on the boot menu.

My comment above was assuming that you had already completed the install and your windows install wasn't being detected by the ubuntu installation and adding it to the grub menu.... If that is correct, then look into the things my comment says.

BUT If you are saying you can't tell or worried about which drive has Windows on it DURING the install? there are ways to figure that out... but the easiest/safest solution:

• Power off the machine
• Just unplug those Windows drives
• Install Ubuntu
• Power off the machine
• Plug your drives back in.
• Boot into bios/uefi and give your ubuntu drive highest(or only) boot priority.
• Boot into ubuntu.
• sudo update-grub - to add windows to your grub menu
• reboot and check the windows install boots when selected.

...worst case scenario, you have to figure out how to get to get them to play nice in the boot menus together... but individually, they will both still boot if the other is removed from the computer.

So, you have Flash Drives A, B, C. Windows resides on A, and drive B is NTFS formatted for overflow data. You are adding drive C and you want to put ubuntu on it so that you can dual-boot.

It really comes down to the size of your SSD's. If A, B, and C are all 1TB each, make 500GB on A windows and 500GB on A ubuntu. If you have a small 128GB SSD for A, you need about 30GB for Linux 20.04 so this might be an issue for windows. In such a situation, I would upgrade to a bigger SSD. 1TB has done me well on my dual-boot with 500 windows, 500 ubuntu.

First, make a backup with Macrium Reflect or similar of A. Make sure you have your windows backup USB. You'll thank me later.

For a dual boot system, you are going to need to put your grub bootloader on A and the ubuntu root on A.

If you have unlimted storage, I would just let the program install it on whatever free portion of your A drive you freed up

If there is a way around this, I am unaware. So run diskmgmt.msc and shrink A by at least 30GB to make space for /root. Run ubuntu on your flash drive, and when it goes into how do you want to install ubuntu, choose "something else". That will load up the Gparted editor in the installer and you will need to format the partitions you are making /root as ext4.

One thing you might want to consider is making C your /home partition, particularly if you have a lot of stuff to store. For a smaller SSD on A (256GB or so), it would make sense. Format it ext4 and proceed with the install.

Gparted is like any partition editor, prone to unintended consequences. Hence the macrium.

See: Questions about Ubuntu 18.04 LTS Install / partition sizing on SSD / HDD system.

You can try this if you use MBR dual boot, after installing both Windows & Ubuntu:

$ sudo su
$ apt install os-prober
$ os-prober
$ grub-mkconfig > /boot/grub/grub.cfg
$ reboot

If you use GPT, add an EFI filesystem at the first partition of Ubuntu when installing. Good luck!