Thank you both for prompt response!
More info: I use multiple external hard drives to back up multiple internal drives, respectively ("HD1 Backup" to back up "HD1", "HD2 Backup" to back up "HD2".) The new OS allowed multiple backup drives but did not allow them to what I try to do simultaneously. So, from time to time, I need to switch backup drives to back up the other internal drive... Which is when I just made the mistake.
I guess in the moment of panic I did not see "add or delete back up drive" button that was cut off by the interface...
Now I got it all straightened out... Thanks again for your tips!