Installing Windows 10 (same product key) on a separate drive in case primary drive fails (multi-boot)

Current Setup:

My machine has four hard drives:

A 237GB SSD C: drive (almost full, nearly 4 years old) with Windows 10. I've got the original Windows 8 installation CD.

Storage Space (Z: drive), for important files. This Storage Space is comprised of two magnetic 1.8TB hard drives.

1.8TB magnetic drive (D:), currently unused and empty. Had some VM hard disk files there but I've moved them to my Storage Space (Z:) to make way for a new OS.

I have a Maximus VI Hero motherboard and UEFI bios.

Background:

I'm worried my C: drive will fail at some point. I need to get up and running again very quickly if this happens. I can't spend any time installing all the programs I need to do my job, especially if there's something urgent I need to deal with. (I realise that something else in my machine could fail, in which case I'd need a redundant workstation. Getting one soon.)

Goal:

I want to install Windows 10 on my spare D: drive. So if I lose the C: drive, I can boot Windows 10 on the D: drive, which will have all the programs I need installed and set up, good to go.

Main questions:

How do I do this?

Can I install Windows 10 on another drive using the same product key or do I need to buy another copy?

Related Questions:

I've dual booted Ubuntu and Windows on laptops before, but that was only one drive. Will I get something like a menu like Grub where I can choose which OS I want? I think I've seen a OS selection menu on Windows computers before.

Do I have to unplug or replug any cables before or after installation?

Will I be able to see the Storage Space (Z:) drive from the new Windows 10 OS?

You may want to look at doing a RAID solution so that the two drives mirror each other.

In this configuration one hard disk fails and nothing happens. Your data is still there as it's duplicated. You won't even notice you lost a drive. If you make another instance of windows you have a few issues. The product key, you've lost a bunch of data and programs unless you take the time to duplicate this every night.

At the very least you should backup your data if it really is this mission critical.

https://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_1