> I expect Alt-Mirror is seeing the destination as unlogged, and so
> unknown, so it has to assume that no files exist at the destination.

You prompted me to try a scenario where the destination drive (D:) had only a few bytes free, and where an exact copy of the source dir structure (C:\Program Files) already existed at D:, but where D:\Program Files was unlogged.

Curiously, this time the Mirror operation worked fine, and with no complaint of "insufficient space at destination". Now I'm puzzled as to what conditions caused the original complaint from Ztree!

With hindsight wish I'd screen-shotted it, although I recall it had a rather messy-looking display at the bottom of the screen, something like:
Error: Insufficient space (786876423876342 bytes required/66562398742442 bytes free) Enter OK Esc Cancel