Rsync difference between two directories to a 3rd directory

Question

How would I go about rsyncing the difference between two directories to a 3rd directory. For example, if I have the command

rsync -av --progress --stats /hdd1/ /hdd2/

I would like to copy the difference between hdd1 and hdd2 to a 3rd directory, without copying any data to hdd2 (as it is a full hard drive). What's the best way to go about this?

Answer 1

Maybe the --compare-dest option of rsync could be helpful. I think

rsync -av --progress --stats --compare-dest=/hdd2 /hdd1/ /3rd_dir/

might work.