Copying database to different hard disk
if you add a second disk i suggest to put data on to different tablespaces. a tablespace offers additional storage and is VERY easy to use:

mkdir /whatever


then in SQL:

CREATE TABLESPACE myspace LOCATION '/whatever';
ALTER DATABASE x SET TABLESPACE myspace;


of course you can also do this for single tables only.
and yes, alternatively you can move the entire data dir and tell the system where the new location is.