peculiarity with external disk

[Dan Lester]

On a High Sierra system, I have an external disk labelled "backup" whose icon is sitting on the desktop. I can open it, and look inside. Seems to be working fine. But when I want to SuperDuper copy "from" the internal disk to "backup" I am unable to select "backup" in the "to" dropdown menu.That didn't used to be the case, and everyone has permission to read and write on that external disk. Even more strangely, that "backup" disk is offered as an option in the "from" dropdown menu.

So strangely, SuperDuper sees the disk, and can read from it, but doesn't want to write to it. Maybe I just have to use Disk Utility to erase and reformat it?

Um, delete that last post. I reposted because it was taking a LONG time to post the first one, and it wasn't clear it actually did it.