How do I zero my hard drive and reinstall OS X?

If your computer’s slowed down and gotten glitchy, it’s sometimes a good idea to zero your hard drive and reinstall your operating system, as this should speed things up and solve any number of issues with your OS. Here’s how it’s done…

1. Gather up all your install discs and serial numbers so that they’re to hand when you start reinstalling applications (zeroing your hard drive deletes everything on it, so you’ll need to reinstall everything. Unless you’re restoring from Time Machine, in which case your should be able to restore your apps intact.)

2. Back everything up. Zeroing your hard drive completely wipes it, so you’re going to want to perform a Time Machine backup and ensure you’ve got copies of your important files and applications on an external drive or in the cloud (preferably both). Don’t rely on Time Machine alone (it’s generally great but like every system, if it’s going to fail you, it’s going to fail you when you’ve deleted the entire contents of your hard drive).

3. Back everything up again, to a different location. We’re not kidding about this completely wiped thing.

4. First you need to boot to the Recovery partition or CD. If you are running OS X 10.6 Snow Leopard or earlier, insert your OS X CD into the machine and turn it on while holding down the C key. If you’re running OS X 10.7 Lion or higher, turn on your computer while holding down Command and R on the keyboard.

5. Head over to Disk Utility. (When you’ve got the Finder menu at the top of the screen, click Go > Utilities > Disk Utility)

6. Go to the Erase tab. Select the drive you want to zero from the list on the left.

7. Click Security Options, then select Zero Out Data from the drop down list if you’re on Snow Leopard or earlier, or drag the slider from ‘fast’ toward ‘safe’ if you are on Lion or later. (Formatting your hard drive is fastest and, while it means the average user can’t get to their files, the information will still be on your hard drive and could potentially be restored. A zero pass writes zeros over all your old data, while seven and thirty-five zero passes write seven and thirty-five zeros respectively. They’re more secure, but take seven and thirty-five times longer respectively.) Click OK.

8. Wait patiently while your computer zeros itself.

9. Exit Disk Utility and follow the on-screen instructions from the OS X Installation Wizard.

10. When you get to the Select a Destination screen, select your newly-zeroed drive. Carry on with the installation as prompted. At the end, you’ll have a pristine new environment to restore your files and applications to. Good times!

11. Begin the slow and laborious process of putting all your files and applications back on your computer. Sigh.

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