Why does shred overwrite the hard drive 3 times with random bits?

Question

Hard disks

I am selling my hard drive, but since it has stored bank account data, passwords, etc. in the past. I want to completely format everything with the help of shred. The disk is very large, so overwriting 3 times randomly, and then with zeros, as shred does by default, takes a very, very long time.
I’m thinking here, and I can’t understand what is the point of overwriting data more than once if the media, in theory, loses all data the first time?

Answer 1

On conventional HDDs, even after overwriting data with zeros (full formatting),
residual magnetization remains (the difference between magnetization 0 and 1),
which, having special tools / special equipment , can be considered, i.e. recover data.
And for an ordinary coolhacker, this is available only in the happiest dream)))

what is the point of overwriting data more than once,

reasonable paranoia = better to overdo it than to bite your elbows later)))

Answer 2

These are the requirements of state standards. Most likely American standard. See the links for more details.
https://ru.wikipedia.org/wiki/Data_destruction
https://habr.com/ru/post/264429/
https://compress.ru/article.aspx?id=16513
https://www.securitylab. ru/blog/personal/Business_...
I agree with SOTVM , such harsh measures are needed when destroying top secret data of state importance, because in this case, the probable adversary - foreign intelligence agencies - can well afford to spend millions of dollars for the sake of only partial recovery of information. Your banking data is worth much less than the cost of recovery (not even guaranteed recovery) after a simple one-time wipe.

Answer 3

shred -n 1- will overwrite once.
https://man7.org/linux/man-pages/man1/shred.1.html
And the question "why" has already been answered by hint000