[Nil]

For  more detailed explanation, see below:

There are a number of common terms associated data and its destruction. Some of those terms include "Erasing, Clearing, Purging and Sanitization".

Erasing is literally just that, you use the "delete" option in your operating system. This would be something like emptying your recycle bin in Windows. All you are doing is removing the pointer information to the location of that file on the hard drive. This can be easily recovered.  

Clearing can also be known as overwriting. This is generally what people are doing when they "write zeros" or other disk preparation methods. While more difficult, it is still possible to retrieve data from devices treated in such a fashion via data recovery labs. Also, this method would NOT touch any sectors of a hard drive that had data in them, but were later marked as bad sectors. Also, solid state media can not be reliably prepared using this method due to the way solid state works.

Purging data is a more intense form is clearing. Basically it's performing clearing functions multiple times.

and finally

Sanitization is a comprehensive process that includes removing or destroying all non-volatile memory (storage media), removing any CD/DVD media, purging and destroying storage media (ssd, hdd, etc).

Contrary to popular belief, degaussing a hard disk drive is NOT a form of sanitization alone. While degaussing CAN leave a disk in what appears to be a sanitized state, often times the degaussing process only damages the electronic components of the hard drive, but leaves the magnetic storage minimally affected. A sophisticated attacker could remove the platters from the damaged unit, or replace the damaged electronics and recover the data.

The only ~100% assurable way of destroying data irrecoverably, is complete physical destruction of the storage media via shredding, incineration, or chemical process.

Isn't there a certain interval of overwrites where the previous data is considered not salvageable? And isn't their software that automatically does this to that "safe" limit? What's your opinion on this tactic assuming the entirety of the disk can be overwritten?

In terms of DoD and Top Secret data, the only destruction level considered assured is sanitization of the media, typically through means of physical destruction.

DoD disk wiping standards state that on magnetic media, 7 passes of random data is acceptable. However, they still would not move media that once contained Top Secret information down to a lower classification AFAIK. Keep in mind that Top Secret data is data that is considered to cause exceptionally grave damage to the security of the nation in the event of exposure. However, consider yourself "the nation" and decide what you think "exceptionally grave damage" is to yourself, and make your decision from there .

Thanks for the reply.
http://www.howtogeek.com/115573/htg-expl...-erase-it/
That article talks about that special number I was referring to, which seems to be 35, but is outdated and irrelevant to modern day drives.

Back to the article that states one overwrite is sufficient, here's a reply to the article:

You have lost the last of your credibility on this one. I have recovery software designed to extract over written disks. The process is basically forensic science against lax security. Using simple filters to subtract the ‘patterned’ overwrite will reveal enough evidence to prosecute in a US court of law. Use a 3-pass pseudo random for most re-use issues, 7-pass if you have stuff you don’t want the average geek to find. Otherwise it CAN be recovered with forensic software. My forensic OS can find things that will make most PC owners blush, or furious – LOL

Makes you skeptical of where you get your information from and what you can actually believe. Because I for one have browsed H2G periodically.

Edit: The problem with all these claims is that there's hardly ever any proof. I'm not saying H2G is wrong because other experts have stated similar things. You don't hear of a government recovering info off of a SSD that's been correctly overwritten besides people telling you they can, such as the comment above. I'll continue to look around and share links if you want.