Drobo

5N2 capacity increase problem

I am trying to add capacity to my 5N2. It had 5 - 6TB drives. I replaced 1 6TB drive with a new 12TB
drive. Waited about 27 hours until all steady green lights, and tried to replace a second 6TB drive, with another 12TB drive. got an error message, you removed too many drives, replace the last drive you removed. That was about 18 hours ago. 5N2 is still in rebuilding mode and does not show on the dashboard. What do I do next?