Linux
rsync -a -v -P --delete -h --stats /media/repat/hdd1/ /media/repat/hdd2
MacOS:
rsync -a -v -P --delete -h --stats /Volumes/hdd1/ /Volumes/hdd2
-a all (-r, -l, -t, -p, -g, -o, -D)
-v verbose
-P show progress and continue partial transfers
–delete delete extraneous files from destination dirs
–stats give some file-transfer stats
Note: the last / after hdd1 is important, so the contents of hdd1 are written directly into hdd2. If you forget, there will be a new folder called hdd1 in the folder hdd2