rsync

connect external hard drive
lsblk -flist drives & partitions
sudo mount /dev/sdd1 /media/ext-hddmount external hard drive
lsblk -f confirm mount
find /<dir>/ -type f | wc -lcount local and external items
sudo rsync -rv –progress /<src> /<dst>start sync process
find /<dir>/ -type f | wc -lcount local and external items
umount /media/ext-hddunmount external hard drive
disconnect external hard drive