Best way to install Win10 on a system with 2 other OS' on separateHDD?

Sorry for that mess of a title...
Right now I have 3 OS' on 3 different drives.

I have win7 (which I never use) on an old 1TB harddrive that is being used for storage.

On a Sandisk 120gb SSD, I have Win10 installed, and this is the main OS I use. When I upgraded from 8.1, this is the drive I installed it onto.

However the Sandisk OS (win10) is rather cluttered, and I need to stop a few processes to ensure the best gaming performance.

So I decided to buy another SSD (ADATA) and have a fresh win10 installation on it. After installing the ADATA SSD, I partitioned it on the Sandisk OS into 2 volumes (1 for OS, the other for game installations)

PLAN/TL;DR: Heres what I want the final setup to look like:
Upon rebooting the system, I want to be able to choose between the ADATA OS and the Sandisk OS.

When Im using the Sandisk OS, Id like the win7 drive to be connected (for accessing the stored files), but when using the ADATA OS, I want the win7 drive disconnected.

How can reinstall win10 on the ADATA drive (in the designated partition) without any boot conflicts with the other drives/ OS?

Should I disconnect all other drives when reinstalling windows on the ADATA drive? How can I change the bios settings to be able to do what I mentioned above?

Is is worthwhile updating my BIOS? how safe is it to do that?




The next section is background info that may help.

BACKGROUND:
I restarted, and booted off an USB drive to install Win10 on the ADATA SSD. Everything worked fine after I installed drivers for my ethernet card and graphics card.

My reason behind doing this is I want to have 2 OS I can boot into. #1 is the Sandisk OS which would be for everyday use, and OS#2 would be on the ADATA SSD and solely be used for gaming. I hoped it would be as simple as restarting and choosing which OS to boot into.


Here's where my problems start. It worked fine for a little while, then after few weeks whenever I tried restarting to switch from the Sandisk OS to the ADATA OS, it wont let me boot into the new ADATA OS. I get an error saying "Disk boot failure- insert system disk and press enter"

I played around with the bios settings a bit, (but I don't remember what I did exactly, was a bit drunk at the time). The one thing I remember selecting was "use the boot settings that work best" (or something along the lines of that). I may have changed the boot order a put to put the ADATA one above the Sandisk and Win7 drives.

After that I was able to get to the blue windows screen where you can select which OS to use. Both worked until the next time I wanted to switch from Sandisk OS to ADATA OS.

When I restarted I got a screen with a QR code saying that startup attempt failed and it would attempt repairs. When it restarts and tries repairing startup, it says repair failed and gives me more options for things like command prompt, use different OS, restart in safe mode etc.

Now if I try to select the ADATA OS from the "use different OS" option, itll just restart and start the attempting repairs cycle all over again.

So what I wanna do now is just reinstall windows 10 on the associated partition on the ADATA SSD drive.


PROBLEM:

Were the issues above caused by conflicts due to the OS on the other drives?

Note that when I first installed a fresh copy of win10 on the ADATA SSD, I accidentally left the drive that had win7 on it connected. (I unplugged the Sandisk OS SSD). Would that cause issues?

When it started giving me errors, I also played around with which harddrives were connected during startup.

How can I reinstall windows 10 on the ADATA drive with no conflicts?

Sorry for the wall of text, and thanks for your help!

-jay95

Unfortunately If you are wanting two(2) instances of Windows 10 on the same pc you will need to buy a new key. You are only permitted one copy per license.

Once you purchase your second copy of Windows 10 - the biggest question becomes how do you want to dual boot it? There are at least three ways to set it up which require different methods of installing the second Windows 10.

1. Always boot from the same SSD and use the Windows 10 boot manager to choose which OS on which SSD to boot from. This way you don't have to mess with your UEFI boot override menu every time you want to boot into the alternate OS.

2. Have two completely independent bootable drives. Doing it this way you must use the UEFI boot override menu to boot into the alternate OS.

3. What I call hybrid between the two, and this is the way I prefer to set mine up. You set up the two SSDs as completely independent bootable devices. After doing that, then you add the other OS to each SSDs Windows 10 boot manager. This way I don't have to use the UEFI boot override menu every time I want to switch the OS - but if one SSD should happen to fail - or I just want to wipe it, the other SSD is always bootable by itself.

To "disconnect" the Windows 7 drive, you just remove it's drive letter in disk management.