select from external table

A simple select from the external table runs for over 15 mins and finally when the result set comes up it is not huge either, just 13 rows are returned and below is the session wait information
Event Wait Information
---------------------

   SID 214 is waiting on event    : external table write
   P1 Text            : filectx
   P1 Value            : 140031948176272
   P2 Text            : file#
   P2 Value            : 18
   P3 Text            : size
   P3 Value            : 99

Why is the select taking so much time? If I cancel the select inbetween the following error is reported

ORA-29913: error in executing ODCIEXTTABLEFETCH callout
ORA-01013: user requested cancel of current operation

please suggest