Select “Mac OS Extended (Journaled)” from the Format pop-up, if that’s not already chosen, then click the Erase button.

When Disk Utility is finished, choose Quit from the Disk Utility menu.

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However, any time you propose erasing your primary data storage, it becomes critical. Also, note that if you’re running Lion or later (Mac OS X 10.7 and up), before making that clone backup, it would be extremely helpful if you were to run the following command in the Terminal (to make sure your user Library folder is visible on your backup): If you plan to install Snow Leopard (Mac OS X 10.6), or any previous system, insert the appropriate Mac OS X install disc and restart the computer.

Hold down the ‘c’ key as soon as you hear the chime, then let go when you see the Apple logo.

Fortunately, there are some easy steps you can take to do the job right. If you only have one backup, remember that when you erase your hard drive, those backups become the one-and-only copy of your data. I highly recommend Time Machine to back up to one hard drive or a Time Capsule, and Carbon Copy Cloner to back up to one or more additional hard drives.

Before I start, note that if you are trying to get the machine ready to sell, you should follow some slightly different directions. Backing up your data is something you should be doing on a regular basis – every single day, for most people. Note that, for the purposes of these instructions, I will assume that you have a backup clone (ie, an exact copy of your hard drive’s current state) made with something like Carbon Copy Cloner.

Installing from older system disks can leave you with a somewhat outdated system, so you’ll want to be sure that you take care of bringing it back up to speed.

If you reinstalled Mac OS X 10.7 or 10.8, this should not be an issue, since you will have downloaded the most up-to-date version.

It’s fast and incredibly secure, using XTS-AES 128 encryption to keep things far out of the reaches of prying eyes.

File Vault is excellent and easy to use, and offers some enormous added security benefit, but it’s not for everyone.

Once your system restarts successfully, you will need to be sure that it is fully up-to-date.

From the Apple menu, choose Software Update, and install any updates that show up.

When it comes to restoring data for apps that are not document-centric, like i Tunes, i Photo or Mail, you need to find and copy the data from your backup to the same location on your new drive.