Did you modify pg_hba.conf? I strongly assume that the password prompt is from psql, not from su. The postgres user must be able to access the cluster without a database, see the default pg_hba.conf:
# DO NOT DISABLE!
# If you change this first entry you will need to make sure that the
# database
# super user can access the database using some other method.
# Noninteractive
# access to all databases is required during automatic maintenance
# (autovacuum, daily cronjob, replication, and similar tasks).
#
# Database administrative login by UNIX sockets
local all postgres ident sameuser
Did you modify pg_hba.conf? I strongly assume that the password prompt is from psql, not from su. The postgres user must be able to access the cluster without a database, see the default pg_hba.conf:
# DO NOT DISABLE!
# If you change this first entry you will need to make sure that the
# database
# super user can access the database using some other method.
# Noninteractive
# access to all databases is required during automatic maintenance
# (autovacuum, daily cronjob, replication, and similar tasks).
#
# Database administrative login by UNIX sockets
local all postgres ident sameuser