When I looked for help how to sell my Macbook Air with Lion from the AppStore privately, delete my data while having a usable OS X for the buyer ready, I got a lot of crap links on Google. As I’m the product manager of FileWing, a PC-Windows program to securely delete data from harddisks, I was also very curious how to securely delete my data on the old mac.
Turns out, it’s VERY EASY. Get your package your Mac came with. In there, you’ll find a USB Stick or a DVD with your original OS X version on it. Put it in. An install app will pop up, and this tells you to click a button to restart your Mac.
In the UI coming up, search the top menu for “Disk Utility”. Select your hard disk and go to Delete. Then pick at least the 7-times overwriting security option and delete your harddisk. After that reinstall from your USB drive or DVD. Boom, done, secure.