What solved the problem:
WD NAS PR4100 Expansion
- Backup all data.
- Replace just the first new LARGER Drive into slot-1; closest to the Power Button. Let the NAS rebuild the drive and finish its task.
- Login to your WD NAS, Select "Storage", Select "Change RAID Mode", Check "Expand Drives". Replace Drives? Select "YES".
- Replace each drive, thereafter, until each drive is "rebuilt". ONE-AT-A-TIME, letting the WD NAS rebuild each one. You will be prompted when to add the next drive.
- The WD NAS now will go into "Parity Test Mode" and correct all drives to the new expansion size. The red light may still be lit on Slot-1 after this. This will take a few hours depending on the size of your drives.
- Reboot WD NAS via Login Screen on your PC.
--Notes:
So, it turns out there are many "solutions" out there. And if you are lucky, you will choose the correct "solution". In my case I went with a solution that wasn't correct. I got my solution in the forums of "Western Digital".
I bought a second NAS and used my 6TB Drives and backed up from my new drives back to the old drives. There was just too much data to lose, it was my cheapest option.
Another side note:
Cloning the smaller drive onto the larger drive does not work. After further investigation, I found that this maps the sizes of the partitions to the new drives as well. That may have been what caused the inability to adapt to the new larger drives. I used a external drive-bay to look at the partitions as the 6TB partition was the same partition sizes on the 18TB partition; of course with a larger end-partition with the extra ~12TB.
I also deleted the partitions on the 18TB drives before restarting this procedure as stated-in-steps above and performed a Quick Format on each as well.