Tyler, it's a complex task, but data recovery is a widely-used and known thing. A simple reformat simply marks all the data as being empty space on the drive, ready to be written over. It DOES NOT remove the data or magically make it disappear. The 1's and 0's, represented upon the HDD platters in what amounts to magnetic grime, doesn't change. The data can all be easily recovered.
A reformat is like going into a library and removing the dewy code from the spine of every book on the shelves. The books are still there and it doesn't prevent anyone from accessing those books, it just makes it difficult to find them. Your OS can't understand until things are cleaned up, and that isn't very hard for any low-watt data-recovery app or person.
There are only two options for completely destroying the data on a drive. The first, and most obvious, is to destroy the drive. For most people, just breaking it and tossing it in the trash is enough. The FBI, NRO, and the military branches, grind their drives up to a powder.
The other way is to use a data-shredder, which is a generic term used for any deletion method that perminantly destroys data without destroying the drive. At the low end, these are usually just tools that over-right files or drives with a single binary digit (1 or 0). More advanced processes overwrite the file/HDD with several passes of randomly generated 1's and 0's. The effect is like taking a JPEG, then randomly arranging the pixels. Then randomly arranging them again, and again, and again, until yer left with nothing but a gray picture with no details or paterns left.
Done deal. Take your pick.
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