How should I erase my hard drive before I give it away?

Conventional wisdom is that reformatting your disk is the right thing to do. And, in fact, I agree with that, if done properly.

What do I mean by "properly"?

Windows (all versions), and even MS-DOS before it, has the option to perform what's called a "quick format". In reality, a quick format does very little except create an empty root directory on the hard disk, and possibly add a label. The rest of the disk is actually assumed to be properly formatted already, and left alone. That's why it's quick.

And that's why it's insecure. Since the rest of the disk is left untouched, any data that may already have been there will remain. Many commonly available disk recovery tools will be able to recover data from a "quick" formatted disk.

So the basic, and common answer is to reformat the disk, making sure to specify "unconditional" format. Depending on the version of Windows or MS-DOS you have, that's typically a "FORMAT /U" at the command line, or making sure that "Perform a Quick Format" is not checked when using disk management tools.

And that's my general recommendation.

But... here's where paranoia sets in.

It's theoretically possible to recover data even after a disk has been unconditionally formatted. It's not easy, and may require special tools (both hardware and software). In general it's a very expensive process. But it may be possible.

It's a common recommendation to use a big heavy magnet to really erase a hard disk. My advice: forget it. Any magnets you're likely to have around the house, even your big speaker magnets, are unlikely to affect your hard disk in any significant way.

An alternate approach is to do that unconditional format several times. Not a bad idea, and if you can, I'd also alternate filesystem types, allocation sizes, and whatever other parameters you have control over, between successive formats. It's not bullet-proof, but it will make that difficult potential recovery even more difficult.

But to really, positively and permanently destroy your data, you need to physically destroy the disk. Personally, if I care enough to go this far, I use a drill press and drill a few holes all the way through the hard drive casing, through the disk platters and out the other side. (OK, ok ... even then it's possible for some data recovery by a seriously dedicated, and seriously well financed, expert ... but it's very much like taking a bucket of shredded documents and trying to put them back together - one heck of a lot of work.)

Unfortunately, that level of destruction also makes your gift significantly less useful.

I'd stick with reformatting.