answer--not physically destroy hdd, for erase data from a hard disk so no one can ever recover it.

Thinking about selling used hard drives.

Recently I bought external 2.5" hard disk drive because my external 2.5" hard drivers have low on free space. Copied all the data to new hard drive from old hard drives.
Now I have number of hard drivers--external 2.5" 500GB, 750GB and a few desktop PC 3.5" hard disks that no use. So I'm thinking about to sell it.

Before I'm selling, I needed to clean up the hard disk so no one can recover my personal data. I don't much have important data though--I have a lot of piece of writings and ideas. But I don't think people will understand what those a pieces of writing is for.
I know this because some of those ideas, I already wrote in my twitter or blogs but no one seems to understand what I am saying. So I don't much care about those.
But I have been learning photography so I have tons of photos which I'd like to erase it.

So I did googling and came across this--in Quora:

How do I erase data from a hard disk so no one can ever recover it? 

Why would you like to physically destroy hard disk which you spend a lot of money for?

Quite surprise to see so many answered with physically destroy the hard drive.

I thought -

Wow...wows!
Nowadays, everyone is sooo rich that everyone suggest to physically drilling, shredding the hard disk.
I'm not rich so I had to completely erase the data before selling.

I know the simple solution without physically destroying it.

I know how software works--because I was a programmer. So I know very easy and simple way. Download some software--bigger size will be better, useless documents or anything from the internet or your not important things like Netflix movie files. And delete all your data. And then copy the downloaded software, documents and all. And kept copy it(same files over and over--make folder to more easier--Windows will make new name for it like below screenshot) until hard disk became zero-ish byte of free space.

screenshot: overwrite files for make unrecoverable

So when the recovery software or professional try to store data from my hard drive, they will only recover those useless data.
It's THAT simple to permanently erase data.
Yes, it takes time. But it perfectly works.

Yes, someone answered what I suggested--overwrite data to remove personal data.

And someone saying that forensic analyse(or something) still can find a bit of information from there. Sure. That might be true.

That's why my solution makes more sense.

Because my personal data was already replaced by thousands, thousands and thousands of files.

That means forensic analyse take more, more and more time to figure out which bit of information connected to the which part of information. It's like thousands, thousands and thousands of jigsaw puzzles to figure out.
And it will be almost impossible because a lot of pieces were missing.

Loren Forslund answered very interesting one and I think it's very good method, too:

My method is using Ubuntu disks utility to break all partitions down and formatting the drive into one “FAT” partition. Now make 2 equal partitions and format them in EXT4 writing over the drive. Now break these partitions back to one NTFS partition over writing the disk. I’m sure if you had one million dollars worth of DOD equipment and 3 months of time you may get a scrap of something, but I doubt anything useful.

People watched way too much TV shows--CSI kind of TV shows.

Especially this part:

I'm sure if you had one million dollars worth of DOD equipment and 3 months of time you may get a scrap of something, but I doubt anything useful.

 - People were watched way too much TV shows

 - and they were way too much talk about this theoretical ideas.

Let's talk about reality.

I told you that I bought a external 2.5" hard disk which has 4TB space--actual size is 3.63TB.
So I put all my old data to this new hard disk..

screenshot: my external hard disk drive's files and folders

Last time, I had a lot of software and documents from the internet. But when I started taking photos, I had to delete those files because nowadays DSLR or mirrorless camera need bigger space to backup.

And other reason is I can always go online to get those software again.  So I don't need to kept in my hard drive.

So in my case, 50% of files(153,598 / 2 = 76,799 files) were probably personal data. But each photo was like 7 to 24 mega bytes only. I have software, downloaded documents, audiobook from Librivox and games and those were far more bigger size than photos.

So even if I have 50 to 70% of files were personal data(because photos I took), 40 to 80% of space were filled with not personal data. That means 40 to 80% of 0.98 tera byte.

That means if forensic analyse find a bit of information, it will quite likely not my personal data. For example, 70% of space were not personal data means:

other not important things - 1,003.52 giga byte * 70% = 702.464 giga byte
my personal data - 1,003.52 giga byte * 30% = 301.056 giga byte

Because my personal data was only 30% chance to picked up by forensic analyse.

And there has more. I have thousands of photos but do you think all of my photos were all very important photos--which I probably use it in my blog.

Photography is my hobby so technically it wasn't very important data.

Even if so, reality is this:

How many professional photographers really use all of their photos they took, do you think?

And how many people do you think they use their hard drive only for their important data?

Even if that's the case, why do they need 1 to 4 tera bytes of hard disk?

I have important documents but most of my documents(which I kept it in text file format--smaller than Word file format.) takes only a few kilo byte.

So I'm sure that a few giga bytes of hard disk should be enough for their personal--important data.

Reality of un-delete kind of software.

How I know this? Because about 10 or 15 years ago, I tried to recover my important data from un-delete kind of software with no success.

I can see the files with so many other files. But I can't recover it because it was already half destroyed--some new files already overwritten some part of those files.

You've known this, too. Good example is 'corrupted file'. Did you recover that corrupted file?

Reality is this:

Even if that forensic analyse find a bit of byte, what can you do with that part of byte?

It was 10 or 15 years ago experience so I have to check. And found this:

Recovering Deleted Files After Formatting: Is it Possible? | Deleted File RecoveryThere are many debates regarding the possibilities of recovering deleted files among computer scientists. All evidence points to the fact that in order to recover files deleted from recycle bin, or any other part of the operating system, they must not have been written over.

In order to recover a deleted file every sector that included data for that file must be in it’s original condition. Overwriting any part of this file with new data will cause corruption.

As you can see, this professional(??) data recovery company saying things were far different than what many people answered in Quora.

Professional data recovery company saying that they can't recover if it's already written over.

But many people in Quora saying that you can recover it--because you can find a piece of information with forensic analyse(or something). So you must do physically destroy hard drive--drilling, shredding, burning and among other physical way.

See the difference.

Even if you recover the part of file, you probably can't read--because of corrupted. But what many people in Quora saying is it can read--recover it.

* If you seen CSI kind of TV show, they can always recover it, don't they?

Yes, you can recover the random data. But do you think that random data is worth a million dollars with spent a few months or a year of work?

That's why I completely agree with what Loren Forslund said:

I'm sure if you had one million dollars worth of DOD equipment and 3 months of time you may get a scrap of something, but I doubt anything useful.

Reality is extremely different than what people were saying in internet, unfortunately.

This is how people kept getting more wrong ideas.

P.S. I found permanent file delete software:
They also explain same thing--overwrite to make unrecoverable.

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