Reading this Japanese article on a kit sold in Japan to build a Z80-MCB, I realized the presence of a possible bug seeing this two images:
As you can see, there is a Z80-MCB without any Virtual Disk or RTC attached and in the resulting screenshot the RTC appears present.
After a quick check I found a little bug in the RTC discovery routine that see a "ghost RTC" when the Virtual Disk is not present.
So I've uploaded a new "sketch" (S221116_R100218_Z80.ino) that fix the bug.
Note that if you have the Virtual Disk already installed, there isn't any real need to upgrade.