usually, it doesn't write just '0's during the sweep, but alternating patterns. Computer forensics can use ultra sensitive equipment to find slight fluctuations on the HDD to determine the previous state of the bit. For example, write a '0' to a '1' and it will still contain a small ghost of a charge. With the right equipment you can pickup what has been erased. Even written over material can sometimes be recovered. Sometimes by the same procedure, or sometimes through HDD data gaps where memory is sometimes dumped. Writing serveral passes of 1's then 0's will help ensure that its unrecoverable, and even then SOME of it maybe recoverabe if the price is right (talking thousands upon thousands of dollars for a small recovery amount, AND not guaranteed). At most I would say it would be fragments of info, but if they pick up the right image from those fragments... well...
If you argue with an idiot he will drag you down to his level and beat you with experience.
I am not a fast writer.
I am not a slow writer.
I am a half-fast writer.