How to delete or clear the external table?
You can only INSERT and SELECT from external tables. To delete all records from it, you should DROP it, delete or rename file on the disk and then CREATE it in database again.
Perhaps the statements for external table manipulation are confusing. To CREATE an external table means to hook up the external file to table definition in the database. To DROP the external table means to release that lock - the file remains intact however.
In newer Firebird versions, this lock is released if there is no active statement running on the external table. If you use some older version of Firebird, you need to DROP the external table whenever you wish to do something with that file from other (non-Firebird) applications or programs.