New Win 10 install doesn't remap OS parittion to C:

I'm not sure how this happened, but I created a dual boot of Windows 7 and 10. When I installed 10 on a second partition (MBR, not EFI) Windows 10 does not remap it's OS partition to C: but stays on E:

This is causing a lot of problems because I need to install various home-grown installation tools that assume the OS runs on C: and has hard coded paths to look for things. (Yes, I know they're poorly written, I don't have control over them).

It's more or less impossible to change the partition letter after Windows has been installed (too many registry entries that have the paths wired in), but How can I ensure that when I do a custom install of Windows 10 to the second partition that it considers the partition to be C:?

Leave the space where you want to install Windows 10 to unallocated (delete the partition there). Install to the unallocated space and let Windows 10 setup create the partition.

I'm fairly certain that's what I did, but I can't remember. So I'll try again.

Thanks for the advice.

So, I ended up going a different route. I wiped my Windows 10 install, made an image of my Windows 7 system with Macrium Reflect. Then I wiped the disk, installed Windows 10 in UEFI mode (with legacy csm support), then I converted my image to a VHD, then added a boot entry for it...

Nice, easy dual boot. The Win 7 seems a little slower, but that's ok.