Hey there, Jumbofile!
Did you unplug the secondary HDD from the SATA ports when you were installing Windows 10 onto the SSD? I'd suggest you try booting with the HDD disconnected and check if you will be able to boot properly. If everything loads up successfully without any errors booting, then you have avoided OS confusion and can easily get rid of the Windows 8.1 on your HDD. For that, I'd suggest you backup all the data from the secondary drive somewhere off-site (external drive, cloud storage, etc.) and
re-format it through Disk Management. If your old OS version is installed on a separate partition, you can simply delete that volume. However, it's highly recommended to backup the data and then transfer it back to the drive after a clean re-format.
This will allow you to boot properly from the SSD and use the HDD for your secondary storage.
Here's
a Windows Install & Optimization guide for SSDs & HDDs that's pretty interesting.
Good luck! Let me know if you have more questions!
SuperSoph_WD