Hello everybody, first, let me thank everybody for their advice. What I did was close the terminal with the device delete-process running in it and fired it up again. It took about 5 minutes of intensive IO-Usage and the data was redistributed and and the /dev/sda/ removed from the list of drives. I am currently running a scrub, which I hope will find and correct any errors caused by the forceful abort of the device delete process. Many people suggested it: Next time I am going to replace one of the older 4TB drives with a bigger one I will use "replace" directly instead of "add/delete".
Yours sincerely Stefan
