"Is it worth getting a USB drive enclosure to try to mount it that way?  I might try pulling the disk out and stick it in another PC as a secondary drive to see if that helps." If you can't see it in BIOs, try unplugging the HDD, reattaching power and SATA cables, but this time plug the HDD into a different SATA header.  If you still can't see it even in BIOs, there may be something wrong with the HDD.  Connecting it to a separate enclosure probably would not help fix the problem.  If you need more storage space, try looking into 2.5" SSD options. 
So, I read up a bit and created a support assist recovery usb to make the change from 1TB HDD with optane to a 1TB NVMe M.2 main drive. My first step was to disable optane prior to removing the physical stick.  I fired up the Intel RST app and clicked on disable optane, and the system crashed tried to reboot. couldn't get a stable reboot to hdd. I figured plowing ahead was my only option, as I couldn't get a reboot to ensure optane had been properly disabled. I went ahead and removed the optane memory stick and installed the M.2 drive and used support assist to reinstall windows on the M.2.  Worked fine, except HDD will now not mount nor visible to any disk program i've tried.  the disk is spinning fine.  I can live without the disk. I had everything backed up and reinstalled programs, etc. 1) can i somehow reformat the hdd to get it to work in it's original bay? are there some settings or software keeping it from being recognized? Should I just remove it altogether? 2) have I bolloxed up something that will make my system unstable at some point? Did I miss something in the reinstall that I should go back and redo? On the plus side, the new drive is unbelievably fast. games load in a fraction of the time and everything feels crisp and responsive.  So glad I made the move, if I can just be sure I haven't messed something important up.
Disable Optane to install NVMe M.2 SSD --> HDD won't mount.