Writable external tables allow only INSERT operations. You must grant INSERT permission on a table to enable access to users who are not the table owner or a superuser. For example:

GRANT INSERT ON writable_ext_table TO admin;

To unload data using a writable external table, select the data from the source table(s) and insert it into the writable external table. The resulting rows are output to the writable external table. For example:

INSERT INTO writable_ext_table SELECT * FROM regular_table;