Interesting. I've read the whole spiel about data being recoverable after being overwriteen many times and from many sources. I've always wondered whether it was true. I mean, I know about hysteresis loops, but given the size of the storage cells on a hard disk these days, it seemed really unlikely that they're not fully magnetised. If data recovery companies aren't going to even try, then I guess that pretty much confirms it's a myth.
I'd be intrigued whether it's possible to recover data on hard disks from 10, 15 years ago which have been treated this way. Back then, the magnetic cells were much, much bigger. What about floppies? I'm guessing the myth must have originated somewhere - although ignorance is a reasonable possibility I suppose.
I don't think anyone claims it's possible to read said data using standard drive firmware, so comparing it to uninitialised memory isn't the whole story. I always figured the theory was that if the cell was magnetised twice, you could tell from the magnitude of the resulting field not only its current magnetisation but also its history. I can see how that might be possible based on magnetic properties of real materials. ( http://en.wikipedia.org/wiki/Hysteresis_loop#Magnetic_hyster... ) However, you'd have to take an analog reading, not a binary one. The idea here being that N, then S magnetisation ought to yield a weaker S magnetisation than S, S.
Strong enough magnetisation will erase that history though, and presumably make the current data more long-lived and random bit-flips rarer. I'm just wondering if the tech used back in the days wasn't sophisticated to magnetise cells strongly enough. (without affecting neighbouring cells)
Yes, I probably am thinking about this too much. I guess that serves me right for doing a physics degree at university. :)
I'd be intrigued whether it's possible to recover data on hard disks from 10, 15 years ago which have been treated this way. Back then, the magnetic cells were much, much bigger. What about floppies? I'm guessing the myth must have originated somewhere - although ignorance is a reasonable possibility I suppose.