Completely Erase Data off HDD

[Radicool]

So I was watching the first season of Mr Robot, an interesting mystery drama series about a group of hackers. I noticed they would completely destroy their HDD and RAM in microwaves, fires, or by drilling several holes into them. This is obviously to ensure there is no trace of recoverable data to safeguard their practices.

I'm wondering why they bother? I thought a Full Format (not a quick format) will write a 0 to every bit of data on the HDD, basically deleting everything? Why waste money on new HDD if they could recycle old ones? Even after a full format, are there ways, maybe with specialist equipment, to read old data?

 

[GhostWolf]

The data on HDDs isn't digital, but rather analog. So, while your processor reads 0 and 1, the real values might fluctuate between these values. The real values of the data might give you hints about information stored in it.
For example, say you had physically set a 1, and then you write 0 to it. The real value is now not 0, but something lower enough than 1, that it is considered 0.
This allows an attacker to guess what the data was before you did a pass on all of the HDD to fill it with zeros. If a value was 0 before the pass, it will still be 0. If the value was 1 before the pass, it now might be 0.4, or whatever arbitrary number that is greater than 0.
This is why there are options to do multiple passes - each one moves every analog value in some direction in the [0, 1] range, and in the end you can't tell what it was to begin with.

 

[Radicool]

Ok thanks for the info