External Tables - Oracle 10g/11g Data and Database Management Utilities - page 61

This table is read only, so if the user attempts to perform any DML operation against
it, it will result in this error:
SQL> delete ext_employees;
delete ext_employees
*
ERROR at line 1:
ORA-30657: operation not supported on external organized table
As the error message clearly states, this kind of table is only useful for read
only operations.
This kind of table doesn't support most of the operations available for regular tables,
such as index creation, and statistics gathering, and these types of operations will
cause an ORA-30657 error too. The only access method available for External Tables
is Full Table Scan, so there is no way to perform a selective data retrieval operation.
The External Tables cannot be recovered, they are just metadata definitions stored
in the dictionary tables. The actual data resides in external files, and there is no
way to protect them with the regular backup database routines, so it is the user's
sole responsibility to provide proper backup and data management procedures. At
the database level the only kind of protection the External Table receives is at the
metadata level, as it is an object stored as a definition at the database dictionary level.
As the data resides in the external data file, if by any means it were to be corrupted,
altered, or somehow modified, there would be no way to get back the original data.
If the external data file is lost, then this may go unnoticed, until the next SELECT
operation takes place.