By default the back-up database is opened in read only mode. You can disable the read only mode if you want.
DefragmentConfig config = new DefragmentConfig("database.db4o");
config.ReadOnly(false);
Defragment.Defrag(config);
Dim config As New DefragmentConfig("database.db4o")
config.[ReadOnly](False)
Defragment.Defrag(config)