You.... reinstall Windows?
Were you the one who installed it in the first place? The easiest possible way to avoid screwing this up is to follow this process:
1. Back up any important data
2. Boot to your Windows ISO usb or disc, select "Custom Install"
3. Delete all partitions on both drives and format them. You can do this in the menu where it says "Where do you want to install Windows?". Just delete it all, format, and power off. Do nothing further with the installer, for the moment.
4. Turn computer off, unhook the hard drive
5. Reboot Windows ISO USB
6. Follow the process it suggests. The only drive it will see is your SSD, therefore ensuring it is your primary drive.
7. Turn system off when finished. Plug in your hard drive.
8. Turn on and finish getting it ready. Your hard drive should show up with a different drive letter
9. Partition your hard drive within Windows (normal boot, not off the installer usb) to your requirements using "Create and Format Hard Disk Partitions" from the start tool.
if you need to move your user folders off of your primary drive:
1. Go to each folder in C:\Users\Username and right click it. Select properties, and in the Location tab, either click move or type the path manually.
2. Hit OK, and the folder should ask you to move its contents and stuff. I generally just double-tap enter when moving these once the path is typed correctly.
Do your other computer stuff, per usual.