I've been asked a couple of times now about erasing hard drives, and whether something will or will not erase data on a hard drive, so I'm going to set the record straight once and for all. The answer to "Does doing XXX erase all the data on my hard drive?" is more complicated than "yes" or "no". It's like asking "Will locking the door at night keep me safe?" It'll keep you safe from curious teenagers and amateur burglars, but not from elite CIA agents or tanks.

The hard drive, in case it needs to be said, is where all your data is stored. Your mp3 collections, address book, bookmarks, etc. are on your hard drive. A typical hard drive is about the size of a small handbook, and contains magnetic platters. The platters are divided into a grid, and each cell in grid can be magnetized or non-magnetized. A magnetized cell might represent a 0 bit, while a non-magnetized cell might represent a 1 bit. Seeing as how many drives can store 320GB to 1TB you can imagine how small each cell is (2'748'779'069'440 to 8'796'093'020'000 cells in the space of a small handbook). If you wave a magnet around near the hard drive, that's essentially going to randomly change the magnetization state of millions upon millions of cells on your hard drive, causing you to lose a lot of valuable data.

For the hard drive to work, there needs to be some sort of mechanism to actually read the magnetization state of the cells off of the platters. Hard drives actually contain a moving head, like the needles of old turntables, that float nanometers above the surface of the platter. The head is capable of detecting the magnetization state of each cell (to read data), as well as change the magnetization state (to write data). If the head actually makes contact with the platter, the platter will get scratched, the head itself will probably get destroyed, and the entire hard drive will cease to function. Remember, the head is only a few nanometers away from the platter itself, and if they touch, you're fucked. The human hair is 120'000 nanometers thick. That's why it's a bad idea to drop, or otherwise shake, your hard drive. Casually transporting your computer around should be safe, but throwing it, or dropping it on the floor is a bad idea.

So dropping your hard drive, or waving a magnet around near it will make it very difficult for you, the average end user, to recover your data, and from YOUR perspective, the data is essentially "gone". But for a mid level technician, the data may still be recoverable. Essentially, specialized equipment would be required. If the head of the hard drive is destroyed, they could replace the head. If magnetization is fucked up, they can still use electron microscopes or do some quantum analysis to make "best-guesses" of the original magnetic configuration of the atoms that make up the hard drive. These process start at $50'000, and become more expensive the more extensive the damage is. So once again, from your point of view, the data is gone, but from the CIA's point of view, it's still accessible.

The reason I make this emphasis is because if you ask me "Will waving a magnet around my hard drive erase my data?" and I say "Yes", you might later come back, angry, saying "WTF? I tried to erase my anti-America terrorism documents by waving a magnet near my HD, but the CIA was able to recover my documents anyway!" Or I might say "No" and you might later come back, angry, saying "WTF? I was putting this cute refrigerator magnet on the case of my computer to decorate it, 'cause you said my data wouldn't be deleted, and now Windows tells me my data is all corrupt." This is why I had to give this long answer, instead of a simple yes/no one.

When you sell your old hard drive (or laptop, with the hard drive included), you probably want to erase all the data on the hard drive in such a way that most technicians won't be able to recover the data, and yet keep the hard drive still in a functional state (so that it still has value to the purchaser). Simply reformatting the hard drive is not enough! There are actually blogs dedicated to people bragging about how they bought used hard drives from eBay, downloaded free, open source hard drive analysis tool, and were able to recover pictures of its former owners naked, or passwords and credit card details, for example.

If you're going to sell your old hard drive, you really need to do a secure wipe of the data. There are free, open source software which you can download to do this, such as Boot and Nuke. There are also commercial tools such as Norton's WipeInfo.

If you want to completely destroy your hard drive (because the CIA or whomever are onto you), then you need to PHYSICALLY destroy it. That means taking a screwdriver, opening up the hard drive itself, extracting the platters, shattering each one as small pieces as possible (a 1 centimeter by 1 centimeter square piece consists of 100'000'000'000'000 nanometers^2, which can fit a LOT of data cells) and then melting those pieces into goo using an acetate torch, for example.