Why do you have to overwrite the data several times for deletion?

Sometimes we get messages from our customers asking WHY we need to overwrite deleted data several times. How is it possible that the command for the first data writing was sent by the operation system, but the disk didn't complete the request? Computers are very strict, not like humans, there aren't options like "maybe" - they always obey commands, so how is it that sometimes the data can be written and sometimes not?

The answer is that the disk also has a smaller computer inside. So, technically data writing is a communication between one computer (your Windows OS) and another one - the chipset of the disk. The chipset of the disk has its own logic, and... here it comes... the chipset of the disk has a cache. Yes, another small cache. We aren't saying that all disks have the same structure (and cache), but many of them do.

So, what happens? For example, you use some program to overwrite the content of the file (securely erase it). But using only one pass - overwriting it once, not several times. The command goes from Windows OS to the disk's chipset queue. In other words, to the cache of disk. In the meantime the program thinks that the overwrite has finished and sends the next command - to delete the file. The disk's chipset sees that there are two commands in the queue - to overwrite the file and delete it. However, if the file will be deleted anyway, the disk logic decides to skip the overwrite. This is because the disk cache was made for speed, not security.

Since there are many different models of disks, as well as manufacturers, chipset versions etc., in the world, and it's impossible to know the algorithms for all of them, security algorithms use repeated data overwriting to exclude all possible chances of disk logic, which can "skip" or "delay" the execution of the deletion. When the data is written again and again, with different data each time, the disk logic will write the data anyway because its cache has limits.

Modern disks are very well protected to ensure that the above situation does not happen. They are very strict and clever, some of them support special protocols for securely erasing data. However, we can't accept the risk in some cases and we need to be 100% sure that the data is erased. That's why security algorithms repeat the deletion of the same data several times.

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