howto copy CalendarServer (content) - use rdiff-backup

rsynced a complete and functional copy of CalendarServer to another drive (and to another file system). afterwards, the server started and didn’t complain but didn’t show any calendar (but still logged in and didn’t complain about anything - just no calendar!).

tryied rsync -aA, rsync -rlpAtgoD (long version - one never knows…), and cp -a - but all to no avail.

finally, i tried rdiff-backup -b - and this worked… why? not quite sure! ideas welcome….