5N2 used capacity still shows 100% even after drive upgrade

In my 5N2, I had 4TB, 4TB, 4TB, 4TB, 3TB. And was pushing the 85% capacity,
Suddenly it jumped from jumped from 85% to 100%.
Ok, I had a 6TB drive waiting which I put in to replace the 3TB.
After rebuilding, it still showed 100% capacity and had a red light on the first 4TB.
I went out and bought another 6TB and put that in, so now I’m
6TB, 4TB, 4TB, 4TB, 6TB.
After rebuild it’s still showing 100% capacity with a red light on the 2nd slot, with the first 4TB.
I decided to run “du -h” on the drive as I had it mounted as an NTFS partition on my linux desktop. The du command shows 12T used, versus the 16.18TB 100% capacity that my drobo tells me.
At this point I suspect something is up with the drobo. I restarted the drobo but get the same thing.
I don’t want to keep buying drives when I don’t need to, and don’t trust the drobo right now to tell me when it actually gets more space.

Is my drobo mistaken, or do I really need to keep buying drives?