The main unit of storage for a PostgreSQL database is the tablespace. Tablespaces are described accurately by their name: they're a space to put tables (and indexes) in. The idea is that every logical disk you want to use for a distinct purpose gets assigned a tablespace name, and then when you create a table, you refer that tablespace to put it there:
$ mkdir /disk/pgdata $ psql postgres=# CREATE TABLESPACE disk LOCATION '/disk/pgdata'; postgres=# CREATE TABLE t(i int) TABLESPACE disk;
Tablespaces are also implemented inside the database using symbolic links, and your OS needs to support them (or an equivalent, such as the NTFS junction) for this to work. Databases and tables are, by default, created in a virtual tablespace ...
