Claudio,
I am not sure if you found out your answer yet...
What is going on is that there is a memo file associated with the database file. The memo file extension is .dbt and the database knows about this file. You need to have the file present in the same place as the database file for it to be open. The reason that you can open the other file is that there is no .dbt file associated with it.
I also think you need to have the .idx or .mdx files also in the same place. These are the index files to the database file, if any where created.
Ken