Question Two of my storage devices stopped working after attempting to put Windows 7 on one of them (was previously Windows 10) ?

[Ivysshin1]

I have three storage devices and wanted to separate them in levels of privacy. One for sensitive activities (like banking), the other for general stuff like gaming for example.
All had Windows 10 installed; I wanted to put windows 7 on one. Before attempting that I booted the pc through this reserve HDD to be sure it was working properly.

First weird thing was that I couldn't boot despite changing the boot order on the bios to prioritize it; after changing to boot from the reserve HDD the system kept booting the main one instead. Seeing that, I decided to unplug the main one and see if then the reserve could boot. It worked.

I then turned a pendrive into a bootable windows 7 installer and restarted the pc to attempt the process. Changed on the bios to boot from the pendrive and w7 installation screen came.
Problem is I forgot my motherboard only supports USB 3.0 and that's the entry for both my keyboard and mouse. Windows 7, unlike 10, doesn't download drivers for you nor supports usb 3.0 by default, so I couldn't type/move mouse on its installation.

I decided to just buy a cheap mouse with old PS/2 plug the next day and give up for now. Plugged the main HDD back and booted the pc from it. All working.

I buy the mouse next day, attempt installation again, but to my surprise the installation wouldn't recognize the HDD. Not the one I wanted to use, nor the third reserve one.
I reboot and try changing booting order to load the HDD I wanted to use instead of the pendrive and to my surprise it says:
''Reboot and select proper Boot device Or insert boot media in selected boot device and press a key''
Tried to boot from the third storage; same message. Despite the fact I didn't even touch this one during the whole thing!

My main HDD is still booting/working fine and when I check the reserve ones from the ''devices and drivers'' menu they seem to be fine. Yet, I can't boot from them.
Worth mentioning I only tried the installation with my main HDD unplugged. And it's the only one I can actually boot.

Specs:
-Storage 931gb western digital WDC WD10EZEX-08WN4A0 SATA (Main storage)
-Storage 500gb HD502IJ (The one I'd install w7 on)
-SSD 128gb NM100 M.2 2280 (third storage)
-CPU Ryzen 1600
-8gb dual channel RAM
-Motherboard Gigabyte Technology Co. Ltd. B450M S2H (AM4)
-GPU RX 570 4gb
-OS on main HDD is windows 10 pro 64-bit

I already tried the obvious solution of resetting bios settings to optimized defaults.

 

[Ralston18]

PSU: make, model, wattage, age, condition (original, new, refurbished, used)? History of heavy gaming use?

My thought being that the PSU may no longer be able to meet the combined power demands of the drives and the combined system.

That said, moving/changing OS's on any given drive or drive combinations is problematic to being with.

Going "backwards" to Window 7 adds another set of potential problems.

Still the PSU is a likely culprit as I understand your post.

 

[SkyNetRising]

My main HDD is still booting/working fine and when I check the reserve ones from the ''devices and drivers'' menu they seem to be fine. Yet, I can't boot from them.
Worth mentioning I only tried the installation with my main HDD unplugged. And it's the only one I can actually boot.

Please show screenshot from Disk Management.
(upload to imgur.com and post link)

I already tried the obvious solution of resetting bios settings to optimized defaults.

That is not an obvious solution. This can change some settings, you're not aware of.
For example - it can change sata controller options and windows doesn't boot anymore.