OP postou um comentário dizendo que o problema foi resolvido. Estou postando o comentário dele para outros usuários que estão procurando uma solução:
Second problem. The same authentication screen came up when I trued to authenticate an attached disk. Password wasn't accepted (as before). So from a terminal / ssh session I ran 'sudo bash' and then used passwd to re-set the root password, which then worked when authentication requested next. So the main problem was the root password authentication. Problem resolved and thanks