Unable to add disk to array

Thanks for the outputs.
So, let's go back to the output log in your first query. SD-A and SD-B should be the the lables of the disk in the array. In your log, it refers no ownership to SD-B so there might still be issues with "accepting" the new disk even if output shows it's present. 

Try running these commands:

dmesg |grep --color sd[a-b]
sfdisk -l /dev/sdb    # I think new disk should be seen as sdb while existing should be sda 
sfdisk -l /dev/sda

If they return disk as failed, or if they return no output, the problem is the new disk, likely. 
One more question, just fr myself to understand this better -- what was the status of the old disk that was removed? 

I'd also like to check if the linux subsystem sees the same status of the array as well .. Can you run array command too? You'll want to confront its output with the following tmsh commands (again - just to confirm again disk status)
tmsh list /sys disk all-properties   # should list both disks
tmsh show sys raid   # it's like the one I gave u last time, just has more output 

Last thing would be checking if the mdadm process is running and in sync -- but I'm not sure if it already exists in version v11, and anyways having a disk in that status will likely result in bad output for the mdadm. 

 grep -e "_U" -e "U_" -e "__" /proc/mdstat | wc -l