Backup and Restore SQL server files and filegroups
Query the backup file with 

RESTORE HEADERONLY FROM DISK = 'D:\sec_file_ndf.bak'


and 

RESTORE FILELISTONLY FROM DISK= 'D:\sec_file_ndf.bak' 


...to verify the contents of your D:\sec_file_ndf.bak file. 

You might have multiple backups in one *.BAK file. If so add the option FILE = 2 to your RESTORE command to restore your most recent backup of the *.NDF file.