The concept of a secondary, external hard drive is pretty basic. Using one increases the long-term storage capacity of a computer. However, looking beyond that, an external hard drive can provide much more than extra closet space.

The following walks through how to create a second, portable, bootable drive. The basic idea is to create a second “computer” without the fuss or expense of entirely new hardware. Additionally, when finished, this external drive will boot any Intel-based Mac capable of running Mac OS X (in this case, Leopard). I have used this portable drive to boot older and newer iMacs and MacBooks. This has the effect of being able to sit at any recent Mac and have the same experience, run the same software, access the same files with little to no hassle.

Materials Needed:

1. External Drive

In addition to your Mac, you'll need an external drive with a minimum capacity of 3GB. This is the minimum requirement for a bare-bones install of Mac OS X. The actual size of the drive should depend on the software you plan on running from it. Because of this small requirement, however, and the increasing capabilities of flash memory technology, it is now possible to create a bootable USB key, but as of 11/23/2008, the functionality of such a drive is limited. An 8GB USB key will work, but it suffers from slightly frustrating performance lag and has little to no capabilities other than running the operating system and a few small specific functions. For this example, I use a Western Digital - My Book Home Edition 500GB External Hard Drive that connects to my iMac via FireWire, and it has been running several of the bulkier software packages (Adobe Creative Suite 3, Eclipse, iTunes, MS Office) without issue.

2. Mac OS X Install Disc

You will also need a copy of your OS X install disk.

Steps:

1. New Drive Setup - Choose Partition Scheme with Disk Utility

Connect your external drive to your Mac and verify that the Mac can access the drive. Using Disk Utility (Applications > Utilities > Disk Utility), verify that the drive is formatted to the proper partition scheme. This can be done by selecting the drive from the menu on the left. Click the Partition tab. Choose a partition scheme. I have chosen 2 partitions for this example because I will not need the entire 500GB capacity of this drive to run software, and I want to use the remaining space as an archive. Make sure to give the partition(s) descriptive names that will indicate to you which partition is which. Click the Options… button to choose a partition scheme. I have chosen the GUID Partition Table option because I want to use this new drive to start up an Intel-based Mac.

2. Start Mac OS X Installer

Insert a MacOS disc into the optical drive of your machine. This should automatically open a Finder window, but if it doesn't, double click the Mac OS X Install Disc icon. Click the install Mac OS X and Bundled Software icon in the Finder window. This opens an Install Mac OS X window. Click the Restart button and authenticate as an administrative user. This reboots the machine from the Mac OS X Install Disc.

3. Install Mac OS X to the new Drive

Once the machine has restarted to the Mac OS X Installer, simply step through the Mac OS X Install process. Choose your main language. Agree to the Mac OS X license agreement. Choose the drive that you named in Disk Utility in Step 1. Under the Options menu in the Select a Destination window, choose how you would like to install Mac OS X. Under the Install Summary window choose Customize and customize the operating system and installed packages as you prefer, and complete the installation.

4. Begin using your “new machine”

After the installation process completes, the machine restarts using the external drive, and you can use it as you normally would a new machine.

5. Choose the drive that boots your machine

Now that you have two drives you can boot from when the machine starts up, you have to help your Mac choose which one you want to use. There are several ways to do this.

Option Boot - If your machine is powered off, you will need to Option Boot your Mac. This means that immediately after pressing the power-on button, you hold down the Option key on your keyboard. You may need to enter your Firmware Password if it is set. This brings up icons of the volumes available to the machine that it can boot from. Choose whichever one you wish to run and continue as you would normally.

Startup Disk - If you are currently running the machine from one drive and would like to switch to another, open System Preferences and choose Startup Disk from the System Preferences window. Choose the drive you would like to restart from and click the Restart… button.