System won't boot with Ext HDDs connected - started in January

Sry, I had hoped to give this issue my full attention today, but Im having other issues on my work PC that I had to focus on today. Likely will be giving any attention I can tomorrow as well. But I'll follow up on the steps suggected in this thread (for the HDD booth issue) either tomorrow or Wednesday. When it rains, it pours T_T

Oh, and I don't know about UEFI on a GPT disk for sure, but If I had to take a guess, I'd say UEFI. But Windows is installed on the internal HDD, not on either of the external HDDs.

I was able to confirm that neither external HDDs are active.

Samuria said:

The bios battery dead would only effect the bios setting ie what to boot from speed hardware etc

The Gigabyte GA-Z77X-UD5H in my desktop PC fails to boot when the battery voltage drops to 2.8V despite the clock being correct

Megahertz said:

Is your Win 10 installed as UEFI on a GPT disk?

As your External HDDs are only for data they shouldn't be set as active (boot able).
Open Disk Management (C:\Windows\System32\diskmgmt.msc) and look if one of those External HDDs is set as active.
If you have one, set it as non active.

As pointed out in post 7