I'm building my new laptop, copied some stuff over via shared network drives, then next day start copying more files like yesterday, except I get error message:

rsync: [generator] chgrp "/mnt/MyDir/MySubDir/." failed: Operation not permitted (1)

After much head scratching and "What's changed? Wrong user? Mismatched trailing slashes? Changed a password?"

Then I realised ...

... I had reversed source & destination paths!

Obvious once I worked it out, but maybe someone else will benefit when they make the same ID-10-T mistake.