Question Bios sees storage drive as main disk, why?

I currently have one ssd for Windows and one hdd for storage, the problem is it'll only boot to Windows if I set the hdd as main drive.

Why is this?
"set the hdd as main drive"

Exactly what does that mean?

If you need 2 drives connected in order to boot at all, it typically would mean that both drives contain SOME components of Windows....regardless of which one is the C drive.

Is it true that you CANNOT boot with ONLY the SSD connected?
 
"set the hdd as main drive"

Exactly what does that mean?

If you need 2 drives connected in order to boot at all, it typically would mean that both drives contain SOME components of Windows....regardless of which one is the C drive.

Is it true that you CANNOT boot with ONLY the SSD connected?
Exactly, I simply can't boot into Windows without my hdd plugged in and set as first boot disk.
 
That's fairly common.

Most likely reason is that you had the HDD connected when you installed Windows to the SSD.

It is fixable with a few commands that I don't know from memory.

Someone else here will.

If you can't wait, reinstall Windows with ONLY the SSD connected.