Or, how Mike wasted a Friday evening in vain.
I bought some new drives for home use, because a drive in my LG NAS went bad. Womp womp.
I got home, and popped them in. As expected: they did not work. They only showed 2TB, despite being 3TB drives. This is expected, because my NAS is old and only does partitions and sectors in the older MBR style.
Well, MBR does not have support for drives > 2TB (technically, 2 and change, 2.2TB usually) for some archaic reason.
What followed, was drive reorganization, and more understanding and explanation than I ever thought I would need to know.