Moving Database -> New Disk

I'm trying to do somethat that shouldn't be so darn difficult - moving a
database to a new bigger disk.

The database is db_domain at /usr/local/pgsq/data/base/db_domain

I want it to reside on /fireball/pgsql/...

I created a dump of db_domain and was able to do initdb on the new
location - but when I try to createdb, of course db_domain is already taken.
If I try to createdb using the -D switch I get an error about it not being
able to find the directory (it's there).

If I don't specify the -D switch, any new database is created under
/usr/local/pgsql/data/base/, even though I restarted postmaster with -D
pointing to /fireball/pgsql/....

So Could someone give some specific instructions on moving db_domain over to
the new hard disk? If there's a FAQ please give a full URL to find it.

Tim