Computer boots to second drive

I have two hdds in my self build; my primary drive and a secondary I use to backup to. For years, there has been no issue.

Suddenly, I started having an issue where computer tries to boot from the second drive, which obviously it can't.

I've tried to edit the boot order from the BIOS. I've ensured that the first boot, after the USB is the correct HD. I don't even have the second HD on the list of boot drives..

However the only way to get it to boot from the right drive is to disconnect the second drive.

I'm at a total loss!