Secure deletion of files in a directory

[Albert]

I have to return a new hard drive in warranty and to delete a lot of 
directories which contain private data. Unfortunately the data of 
S.M.A.R.T are erased too, if I use badblocks -w to overwrite the 
_whole_ drive.

I know there is a possibility to shredder a file with gpg, but not a 
directory. What do you recommend?

Albert