The strategy of closing a sprint before removing a superfluous board causes the warning to "move incomplete cards to backlog". I tried to hack a new board filter to return zero cards on the board, but when I tried to close the sprint (to remove the board) the sprint cards were still classed as incomplete when I tried to close the sprint.
I'm concerned that doing so will affect my other scrum board and move all of those to the backlog in that board. Is the backlog of one board related to a backlog of another? IE will moving cards to a backlog of a board (from deleting the sprint) cause cards to be moved on another board? Can I safely close and drop this board without affecting card statuses on another scrum board ensuring they don't suddenly go to backlog on board 2?