=# CREATE WRITABLE EXTERNAL TABLE unload_expenses 
   ( LIKE expenses ) 
   LOCATION ('gphdfs://hdfslhost-1:8081/path') 
 FORMAT 'TEXT' (DELIMITER ',')
 DISTRIBUTED BY (exp_id);

You can only specify a directory for a writable external table with the gphdfs protocol. (You can only specify one file for a readable external table with the gphdfs protocol)