You don't want to use dd. That is for working on 1 file or stream, not on a whole filesystem.
rsync is designed to do what you want but as the previous poster has stated, and as my tests have shown, it's not the fastest. That's because it's for doing something like this: "Ok, I'm looking at file A. Is file A on the destination? If so, is it newer, older, the same?" Etc. rsync is a bit complicated because it is meant to be run more than once... like the name says, it's for synchronizing two locations.
For doing the sort of thing you want, I have found a tar copy to be quick, easy, and reliable. Tar knows about hard links. Tar knows about devices. Tar handles almost any situation you'll run into in your filesystem (except really long paths, and, if you're not using Gnu tar, you may need to be wary of putting a / in the beginning of your pathname).
Anyway, I've had 99.98% success for the last 20 years doing by doing this:
cd /my/source; tar cf - subdirectory | (cd /destination/path; tar xf -)
...The subdirectory you want to copy will show up in /destination/path .
If you like to watch your progress, you can use "xvf" instead of "xf" in the latter portion of that string.
...my 0.02% failures have come from really long file paths... :-(
Tar will not guarantee file integrity. That said, as long as you don't see any error messages, I've found it to be very reliable. It will keep permissions and ownership properly.