Using Finder’s copy and paste is a bit daft. There’s nothing nuanced in its operations, or if it is, it’s not user friendly enough for me to have noticed.
Every time we tried, it would fail after several hours. Re-trying meant that it would start from the beginning again, and you guessed it, fail after several hours again. Long story short, it was going nowhere.
Thankfully, after a little searching looking for a RoboCopy type tool, it turns out that OSX has rsync installed. Phew.
The following command copies the source to destination, ignoring any files that have already been copied, and the source not subsequently updated.
rsync --update --progress -va <source> <destination>
Done. Files copied as expected.