In 2.1.2 SuperServer, any data written to external tables are visible to other sessions.
However in Classic, this data is not visible. It seems to be cached and written to file eventually, when this happens it becomes visible.

Super Server behaviour:
We created a new external table.
In session A we inserted data with no commit. The external table file was showing 0 size.
Started session B and selected data from the external table. The data was returned, and this triggered the a file flush and the external table file now shows the data (viewing the file in Windows).
Committed session A and data visibility still behaves correctly.

Classic behaviour:
We created a new external table.
In session A we inserted data with no commit. The external table file was showing 0 size.
Started session B and selected data from the external table. No data was retuned and the external table file is still 0 size.
Select from external table using session A or Commit or disconnect session A and the data is flushed to the file. It is now visible from session B.