How do I copy a Mac volume using cp?
Use rsync for this purpose since cp cannot faithfully reproduce all the files that exist on OS X. The benefits of rsync over ditto and cp are:


interrupted transfers can restart easily with very little cost. 
restarts even resume part way through a large file. 
file exclusion and --dry-run allow easy testing and iterative thinning.


The simplest way to use rsync and preserve most things (permissions, symlinks, edit dates, etc) is to use the -a or --archive flag to archive. To see your progress use -P or --progress

Rsync works as:

rsync options source destination


In your case I would recommend the following:

rsync --archive --verbose --one-file-system --human-readable --progress /from/ /to


Or in short:

rsync -avxhP /from/ /to


Mind the lack of trailing slash in the /to location.

see comments for more useful flags