OS won't boot from C:

Hello, so my OS won't boot from my C: drive when I specifically ask it to.

I have two HDDs, B: and C:. Both have the same version of Windows 7 installed on them. I want to format B: and copy the contents of C: onto it but I can't do this as B: is currently running Windows. So I should just be able to change the boot order or specify in my boot manager that I want to load from C:, but neither of these work. No matter what, even if I specifically tell it to boot from C: it always loads from B: instead.

How can I get it to boot from C:?