If the HDD is already formatted, it should still work even if you reinstall Windows, though there is a small chance of permissions issues, but that can usually be solved by moving files so you can delete and re-create your folders (as long as you're careful, you won't lose your files). Also, if you have applications installed to the HDD, you may need to reinstall them to restore registry entries they need.
Like MauveCloud said, as long as your HDD contains only data then you should be ok. If you've installed any programs onto the HDD then they will need to be reinstalled. If you have any programs on the SSD(OS) drive and you've pointed any data content to the HDD then you'll need to remember to change the default data location to the HDD.
Remember to backup the C drive regularly using e.g. imaging software e.g. Acronis True Image or Norton Ghost - this is a great method in case of failure/data corruption. Then, it's a simple matter to reconstruct the drive's contents in minutes!
If and when the SSD dies, that's all you need to replace. The HDD will still work just as it did with the old SSD.
However, if you install any Windows games or applications to the HDD and then install a new SSD, you will need to re-install those games and applications after re-installing Windows on the new SSD.
That's because when you choose to install software on to your non-Windows HDD, some support files and registry keys will still get written to the Windows drive (the SSD). This is unavoidable. When you re-install Windows on a new SSD, those support files & registry keys will no longer exist, so any applications related to them will not run -- therefore you have to re-install those applications.