Oh... Sorry for the delay in getting back here.
Actually, that idea is one just crossed my mind with thought that is there any chance to reduce some overwrite passes though HDD wipe must have done by a lots of overwrite pass.
Also I didn't think how it could be verified as expected. So, I said.
After I see new reply, I looked for "secure deletion" and found paper which introduce "The Gutmann method".
"Epilogue" in this paper say "For any modern PRML/EPRML drive, a few passes of random scrubbing is the best you can do.". He made "The Gutmann method" and said that.
Also I got another paper in 2008.
This paper show the probability of recoverability one bit (not one byte) using MFM.
After read this, though process in this paper may be wrong, I do not really think there are any chance to recover data securely erased using one pass with random data.
If you want wipe data truly, you may crack your HDD by hammer, too.
I'm stick to one pass overwrite now.
Thank you for reply. I don't feel like I need to count on what I said.
P.S. You may rename file as "153"(any random digits and make no extension) before normal/secure erase private data(software key, backup code or so on) to make sure no clue via filename in MFT.
Move each file to root dirctory in drive may good, too.