By-the-way I cloned my boot volume using TTS without a hitch.
My inability to do so turned out to be a bug in TTS.
When copying a volume group, there can indeed be a "misunderstanding" between TinkerTool System 6 and macOS if the volume group comes from an APFS container that holds more than one group. In this particular case, macOS cannot clearly identify which of the groups to copy.
Of course I then had to rename the cloned drive so I could tell them apart.
How did you determine which volume was your clone? Neither
QuickBoot nor an option boot gave me any indication of which of my identically named volumes was which. (I'm going to submit a feature request.)
