Remember that when you're pretty sure you've deleted that file forever, you probably haven't.
Hard drives store data all over the place, then create a list of pointers, letting their internal software know where to look for the data on the disk. Kind of like a phone book it's a list that says, “This program lives here, and this one lives here”. Deleting a file usually means you've deleted any reference to it in the phone book. You haven't actually deleted the file. All you've done is taken the listing out the directory on the hard drive, so now the drive doesn't know how to find your data. But, just like if you take your name out of the phone book, it doesn't mean you've moved. If someone knows where you live, or can recognize you by sight, then they can still find you.
The same goes with files on your computer. Each file on a drive has a header. This is to describe to programs what kind of file they're about to encounter. Since this header is a description of the file – what type and what the file name is or was, programs can be written to look for files, without the phone book, or directory listing on the hard drive. Until a file is actually overwritten, there is software available that, for a limited time, can still find it.
Thanks for reading!
Until next week, I'm Computer Dave
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