postgres

Hi ,
i had create a database named "mydata" with postgres 7 and i had no problems
with it.Then i copy mydata/ to another computer which is well configured.But
i can't access to my database   the error is that the database mydata is not
in the system catalog.

an another error i had after trying to fake postgres is that the oid 12345
dissapeared from pg_database.(i had create a db named mydata  and then copy
the tables and indexes in the directory mydata pgsq pgql create)

do someone know how to copy a db fro one comp to another ?    (i'm under
redhat 7)

thx