Close........it's not the separate drive that's not recognized, it's the empty space on the separate drive which I was attempting to use to create a new partition which isn't recognized. The other partitions work fine within Mint and the drive is, therefore, recognized by the bios. The only partition manager that would even work properly is the KDE one which just tells me the partition table is gone and won't allow me to do anything further with it.
They are SATA drives. I'm 99% sure that the separate drive is where I've set up the boot partition (MBR) and I know that I didn't delete it when I attempted to create the new partition, but rather think that something went wrong when attempting the installation of the second OS.
I'm tempted to use the weekend to reinstall Mint and redo all my personal settings on everything, etc. but am a little worried that the remaining usable partitions on the separate drive will be lost leading to lost data.....unless of course I go and grab yet another 1TB hard drive to back everything up onto. I was hoping to not have to part with any money on this.