system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When securely formatting a drive, what is the difference in terms of security between a single pass of 0s and multiple passes of 0s?

Wouldn’t one pass be enough? How can data be recovered after a single pass?
gpt
On older spinning platter magnetic media drives, where each "bit" of information is just a small area of the magnetic platter that gets magnetized or demagnetized by the read/write heads. But if you think of how the region gets magnetized, it isn't a consistent amount - the head's EM field only has a very short time to induce the corresponding field in the platter surface. Think if you will like coats of paint. 

If you paint a freshly drywalled wall, that first coat of paint is maybe 80% but you can still see some thin areas where the drywall colour is coming through. Give it a second coat, you're up to 98% and after a third coat its 99.999% coverage. Same thing with the magnetic surface. Write a "1" once and its maybe at 80%, 2nd 98% and so on. Conversely, writing a "0" might only get it to 5 or 10% charge. A "1" AFTER a "0" might be 75%. With careful calibration and the right recovery tools (hardware AND software), you can figure out what two 1's after 0's looks like for example and work backwards and if you're lucky you can rebuild what a file might have looked like. 

But this level of forensic recovery is really expensive and only worthwhile if its like FBI state secret evidence stuff, or the crypto keys to your fortune are on that drive. Writing successive zeros gets those bit regions closer and closer to 0% charge. But, if you had to do it, do random 1's and 0's not all just 0's. Buries the data under random noise, makes file recovery much much harder.

But if you only do a single pass of just 0's, there are some prosumer tools that might have some success.