Computers think in ones, zeros
Computers are peculiar creatures; while humans think in English, Yao or whatever language, computers think in ones and zeros.
On the surface, computers are made to costume themselves, yet under the hood, they are something else. That camouflaged look is called user interface.
On application level; QuickBooks or say Result Management System, when you hit the delete command, the database (Accounting systems are essentially database systems) does not remove the selected data. What QuickBooks or Sage does is to strike out or mark that piece of data as unusable.
The reason is simple; it is important that such systems keep records of people who log into them and trace their activities.
It is important for security reasons, for the system to keep history of anybody who ever deleted data so people with bad intentions can be exposed. Better still, for those that made genuine mistakes, their cases should be pleaded for.
These are rightly called audit trails. That should be self-explanatory; system auditors use these to trace fraud or unintentional mistakes.
This is essentially the automation of international manual accounting and banking practice; errors must crossed out and signed for.
On operating system level; I am talking about Windows, Android or iOS, it is very much the same. When you delete a file from a computer, that piece of data is not actually removed entirely. The system marks that data. What that means is that another piece of data can be overwritten on that space.
The overwriting does not take place overnight as the storage facility may still have completely empty spaces. This means that files that you delete on your flash disk or indeed on the hard disk are still there but you cannot see them.
Does that scare you?
Any good recovery software can ‘unmark’ those spaces and dig out the deleted files. This is possible even after formatting the hard disk and installing a flesh operating system.
What is the philosophy of this story? Number one; the world delete is not what it seems to imply. Surely the files seem to disappear to the ordinary person but visible to some whiz kids with some technical spectacles.
Number two; if you have secrets on your computer and you want to sell it, don’t just delete; remove the hard drive and crush it to dust and throw the powder onto the deeper waters of Lake Malawi.
That said, there also pieces of software that can incineratedata. Otherwise, don’t just delete!