I forgot to mention that FMS15 version is 15.02.206
Current version is 15.0.3
To answer my own question then:-
The help article:-
'Setting up the container data folders' is quite helpful. Having read this I don't think that the greyed out path fields is a bug. There are a couple of things.
After you upload databases to an additional database folder and open the databases, you can’t clear the Enable container data folder 1 andEnable container data folder 2 checkbox or change that additional database folder path. To stop using that additional database folder, you must first close all the databases in the folder, then clear the checkboxes. If you change container folder, you must manually move the data from the old folder to the new folder, then select the location of the new folder.
If you change the container folder, you must manually move the data in the old folder to the new folder. After doing this, enter the new path.
•If you enable an additional database folder and select that additional database folder as the group folder for an administrator group, you cannot clear the checkbox for that additional database folder. To stop using that additional database folder, you must first change the group folder for the administrator group. See Adding administrator groups.