If the US government states you need to overwrite a HD with 1's for so many passes, there is a good reason for it. I would asume the head of the drives reads a few magnetic particles to figure out if the byte is a 0 (under a specific reading) or a 1 (over a specific reading). Playing with the actual raw data might tell you what it used to be.
For example if the last recorded bit was 0 and you record a 1 over it the head might read 70%, write a 1 again and it might be 85%, do it again and it might be 95%. HDs don't write each byte to 1 specific particle, they do it to a small patch of particles and the DSP processes the readings of the head to figure out if there are enough values averaged to be a 0 or a 1. With magnetic recording knowledge, some very sensitive equipment, the desire to do so, and some error correcting routines,you could probable figure out what was there before the last write or two. You are dealing with digital data recorded in an analog world.
As far as old laptop drives, I have picked up quite a few semi functional laptops from ebay and other places. Once you get them working and look over the HD you find people have left all their personal letters and data on the drive including bank accounts and other sensitive stuff. I look for drivers for old hardware, save that then reformat the drive and reinstall the OS. On the few systems I bother to pass on to others I will take the HD and do pattern writes and then reinstall a fresh OS just in case I left something personal on the system and forget about it.
For example if the last recorded bit was 0 and you record a 1 over it the head might read 70%, write a 1 again and it might be 85%, do it again and it might be 95%. HDs don't write each byte to 1 specific particle, they do it to a small patch of particles and the DSP processes the readings of the head to figure out if there are enough values averaged to be a 0 or a 1. With magnetic recording knowledge, some very sensitive equipment, the desire to do so, and some error correcting routines,you could probable figure out what was there before the last write or two. You are dealing with digital data recorded in an analog world.
As far as old laptop drives, I have picked up quite a few semi functional laptops from ebay and other places. Once you get them working and look over the HD you find people have left all their personal letters and data on the drive including bank accounts and other sensitive stuff. I look for drivers for old hardware, save that then reformat the drive and reinstall the OS. On the few systems I bother to pass on to others I will take the HD and do pattern writes and then reinstall a fresh OS just in case I left something personal on the system and forget about it.