Problem:
V4 --> V5 migration key note.  Be aware that we have 
changed the way that backup and restore external files 
between v4.x and v5.x.

Solution:
In InterBase version v4.x when you backup a database 
with external file relations, GBAK will import the data into 
the backup file.  When you restore the database
the once external files are recreated as internal tables.

In InterBase version v5.x, GBAK only backs up the external 
file definition and ignores the actual data in the external file. 
Thus, when you restore the database there are no records 
stored internally.