Hrm...
So I had a disk fail (disk2 - 6TB); physically installed the new disk (14TB) in an open tray, leaving the 6TB attached (array is down), and cleared the new 14TB.
I then replaced the 6TB in the disk2 entry for the array with the new 14TB and started the data rebuild. Seems normal.
Rebuild finishes, but new 14TB disk2 is now reading "Unmountable: Wrong or no file system", yet shows btrfs as the FS for the entry.
I've searched every sequence of terms I can think of, but can't seem to find anything covering this particular sequence of events.
What really worries me is that after the data rebuild has finished, my docker img file is still not found (since I didn't configure that correctly to be on my ssd pool for containers at the start).
I have NOT done the "Format all unmountable disks" on the array page.
Have I lost data in this rebuild or is there something I'm missing to get this disk online correctly?