How to Erase Your Macs Hard Drive Before Selling It (OS 10.4)

Hannah, the manager of our Burlington retail store recently decided to upgrade her Core Duo black MacBook to a Core 2 Duo black MacBook. First, she used Migration Assistant and a firewire cable to move everything from the old Mac to the new Mac. Migration Assistant is included with OS 10.4 – it can be found in the Utilities folder which is nested in the Applications folder.

Next, Hannah needed to erase the old MacBook’s hard drive, and reinstall a fresh copy of the operating system (OS). The OS installation disks that came with your Mac (sometimes called the Software Restore disks) or a OS 10.4 Install disk is required to do this.

Most people simply install a fresh copy of the OS over the old copy of the OS. This has the effect of overwriting and obscuring / erasing their old data. To do this, pop in the OS install DVD that came with the computer (or a purchased copy of the OS), restart the Mac holding down the “C” key, and follow the onscreen installation instructions – making sure to choose “Erase and Install” when it comes time to actually install the operating system.

However, Hannah wanted to be sure that her old data was really erased. Thus, she decided to first erase the MacBook’s hard drive before installing the new OS. Again, pop the OS installation DVD into your Mac’s optical drive. Restart the Mac while holding down the “C” key. This time choose “Open Disk Utility” on the menu bar from the Installer menu. In Disk Utility, select your Mac’s hard-drive icon in the left column. Click the Erase tab, then click the “Security Options” button.

Now you can choose to Zero Out Data, which provides good security in minimal time, 7-Pass Erase, which writes data over the entire disk seven times (and takes seven times longer than the Zero Out Option), and even a 35-Pass Erase option – which would take a very long time, would be extremely secure, and is great for very paranoid people.

Why the different security options? When a hard drive is “erased” most of the time the old data is not really erased, but set aside to be written over on an as-needed basis. If you never actually write over the old data, it is still there – ready to be recovered by scheming people.

To truly erase a drive so that no one else can recover old data, new data (random data, a meaningless stream of 1’s and 0’s) needs to be written over it multiple times. When erasing a Mac’s hard dive prior to a new OS installation, most people will be fine choosing “Zero Out Data,” and then re-installing the OS. However, doctors, lawyers, accountants, and spies should probably choose 7-Pass Erase before reinstalling the OS.

It does take a couple hours to securely erase your hard drive and install a fresh copy of the OS – but if you have sensitive data, it’s worth the trouble.