Disclaimer: By deleting, I don’t mean sending files to the “recycle bin” or “trash” folder. I mean getting them off your computer entirely. Also, this article does not take into account SSD’s, which have a different operating mechanism.

Basics

I wasn’t quite sure of the best way to start writing this, but I hope this introduction helps put things in perspective for you. The thing is this: Data is ultimately just a collection of 0’s and 1’s, arranged in different ways, to produce what we know to be electronic files, whether it be audio, visual, or both. These 0’s and 1’s are abstractions (things put in place to prevent you from having to know those inner details) of electrical impulses. Electricity is matter; it has mass, and occupies a volume of space that cannot be occupied by something else at that particular point in time. Just hang on to this piece of information first, it’ll make sense as you read.

Now, following something called “the law of conservation of mass”, matter (which has mass) can neither be created nor destroyed, but can be converted from one form to another. Basically, you can’t bring something out of thin air, and you can’t make something vanish into thin air. You can only change it to something else. An example being converting trash to smoke by burning. Looking at it from the purely scientific standpoint, you can’t just press a button and wipe something out of your hard drive so magically.

Something to think about

If you doubt what I’m saying, let me ask: Have you ever wondered why it takes you a couple of minutes to send large files (say something of about 5GB), but that same file can get deleted within seconds? Have you also tried moving a very large file from one folder to another, but then you notice that it also takes a few seconds (just about the same amount of time it takes to delete a file)? If your answer to both questions is “yes”, then you are on the right track with this article.
Moving and deleting files work pretty much in the same way, except that with the former, you still have it, and with the latter, you suddenly don’t have it anymore.

We are assumedly making process. To paint a picture for you, I will give an analogy of how “moving” works. Let’s say you have a piece of jewellery that you really like. You keep this piece of jewellery in a box with a label or tag called “jewel box”. Now, let us say that you want to “move” this piece of jewellery to another box somewhere in your house called “safe box”. What you would normally do would be to take out the jewellery from the “jewel box”, then put it in the “safe box”. This is how you would do it, but this is not how a computer would do it. What a computer would do would be to remove the “jewel box” tag, and replace with the “safe box” tag. Just a quick tag/label change.
Likewise, using this analogy, “deleting” or throwing away the jewellery would just mean throwing away the “jewel box” tag, with the box itself still exactly where it is. Assuming you asked someone else in your home to retrieve your jewellery from it’s rightful place i.e the “jewel box”, the person would simply go search for it, then come back and tell you that there isn’t such a box, because there is neither a box labeled nor tagged “jewel box”.
This is exactly how it is for computers. And it works this way because of speed and ease. You probably wouldn’t be happy if you had to wait several minutes for something to get deleted.

Take-away

When you store a file in your hard drive (external, internal, or whatever), 0’s and 1’s are arranged in certain ways, in a certain memory location, and your computer keeps what seems like a table of contents in a book. With that table of contents, you can easily access the location (or page number) once you know the name of whatever you are looking for. When you delete, you only erase a listing from the table of contents, but the book is pretty much the same, and anyone diligent enough can locate the pages if they search thoroughly.

It is important to note that if the process stopped at this, hard drives and memory devices would be full most of the time, as devices never really delete data. However, this isn’t the case. Remember earlier on when I talked about 0’s and 1’s being abstractions? Well, the inaccessible data can be overwritten (Look at it like repainting a wall). When it is overwritten, the new information is written on top of the older pages that are no longer listed in the table of contents in the book analogy, and the table of contents is automatically changed to represent the new information, accurately. It’s important to remember that a replaced chapter might not fill up the same number of used pages previously used, and in such a situation, evidence of what was first written would still be there somewhere, and be available to anybody who is nosy enough to check.

TL;DR

Deleting doesn’t really delete. It just moves stuff into a location that neither you nor your operating system can ordinarily access any longer, because there is no way of knowing how to access them. And only on overwriting with new information, can a hard drive truly let go of something that is supposed to have been deleted. Even then, some forensic specialists have ways of recovering some of the data even after dozens of layers of overwriting have occurred. How they are capable of this, I do not yet have the knowledge of. If you really want something cleared off your hard drive, your best bet is to destroy it with a hammer, or melt it in acid. Otherwise, you can just encrypt the hard drive heavily, so that even if the said digital information was accessed, it would be utterly meaningless to whoever got a hold of it.
There is one last option worthy of note though; you could get special software tools for clean deletions. They work by rewriting over the old data with random data. This isn’t particularly recommended -for security reasons.

In Conclusion

I have to mention that this process works differently across different types of hard drives, different operating systems, and different devices. For the most part, though (especially with Windows users), this is how the deletion process works.