….according to Apple that is not a bootable copy of the OS. Did you ever install MacOS on the drive you now use for Time Machine backups?
No….see reply above to artie.
[If so what you are seeing may be a Recovery Drive.
That’s what I’m coming to believe, but why would the icon clearly be the Time Machine icon? Does a Recovery Drive have its own icon? Could it be that Apple is saying: “You may have a system folder on the Time Machine drive, and you may have selected it as a restart option, but we’re not allowing it. Instead we’ll use it as a Recovery Drive.â€