I often add data, and it tells me to merge directories and skip files I’ve already copied. There’s space for the new incremental data, but I can’t ignore the message about insufficient space.
AIUI you currently can’t. See:
While I can agree the way dolphin handles this is far from perfect, it’s also a really niche problem.
If you have directories you often want to keep in sync I’d suggest using a tool for that specific use case. I’m quite particular to freefilesync (https://freefilesync.org/).
or syncthing works really well, esp for keeping mobile devices in sync with the desktop.
Oh well, I’ll have to work around it, e.g.: rsync -avX --ignore-existing --progress /source/ /destination/. Thx