I have had a similar problem with at least one of the DB implementations. I can't recall if it plain DB or one of the one of the others.

The problem I found was that, to use your example, DB was not trying to open 'ECO' but 'ECO.pag' and 'ECO.dir'.

IIRC the specification of DB defines an API but not a file format. There is no promise of compatability between inplementations.