"dd" is a much better command that "cp" for this. See the following for more info and a good background as to what is going on:
https://wiki.archlinux.org/index.php/Disk_Cloning
The sections on "Cloning a partition" and "Cloning an entire hard disk" are the bits you want. Also take a look at the bit about the MBR - useful if you want a bootable drive after the cloning if you didn't take care of it when you created the partitions on the new drive.
Andy