Drobo

5N Still Showing FULL

One of our Drobo5N with dual redundancy, had NAS Drives: 4TB 4TB 4TB 8TB 8TB (with what I thought was about ~20% freespace last time I remember looking).
Added some movie files over the fall.
Two days ago, Dashboard indicated that the 4TB drive in Bay0 needed to be replaced due to low usable space.
I replaced the 4TB drive with a 10TB drive.
Per the Drobo calculator, 4/4/4/8/8 was at ~10.89TB available space, now with the new 10TB drive it jumps to ~14.52TB.
After the Drobo went through 30hours of re-distribution of data, it now says that the 4TB drive in Bay1 needs to be replaced due to low free space.
How can it still show FULL when I just added 3.63TB of space?
Not looking forward to another $300 10TB drive just to find out it still thinks it’s FULL.
Not very interested in going from dual to single drive failure either…
Thanks for any insight,
Mark
PS: we have five Drobo "5"s; two 5Cs, two 5Ns, one FS