Moreover, my understanding is that modern magnetic storage is, during normal operation, functioning very close to the limit of what the media can store, and what we have the technology to read; the concept that electron microscopes can recover overwritten data was only true decades ago. With this in mind, even a single-pass overwrite is likely to be sufficient for most purposes -- if we had the technology to recover data after it's been overwritten, we'd be using that technology to double hard disk capacities.