Tutorial: Boot a Mac From an External Hard Drive

Having an external hard drive that’s set up to boot into Mac OS X can come in handy.

If your computer isn’t working properly, you can boot from it and do diagnostic testing.* Some kinds of backup process work best when you’re not booted from the drive you want to back up.* If you want to upgrade the hard drive in your Mac, an external boot drive can help.

For me, running a computer workshop, it also lets me quickly install custom configured versions of Snow Leopard with minimal hassle.

This article will take you through the basics of setting up an external hard drive so you can boot from it.

Tools Needed:

• A Mac computer of your choice:
  This process will work on any current model Mac and many non-current models as long as they have a connected DVD drive and the right external ports for the external hard drive of your choice.If you’re setting this up so you’ve got something for when things go wrong, it helps to be doing this before things go wrong.* If you don’t, whatever is wrong might stop all this from working properly.

• An external hard drive:
  I like using the LaCie rugged 2.5” drives for this sort of thing.* They’re in a case that I don’t mind getting knocked around a bit and include USB as well as FireWire 400 and 800 connections.Since I work on a variety of Macs, having the interface choices is really useful.* If you’re only using this on one Mac then feel free to use the interface that works best for you.

• Snow Leopard Install DVD:As you’ll be installing a new copy of Snow Leopard, you’re going to need the discs to do it.* You should be able to use the ones that came with your Mac (or the upgrade you purchased) without any problems.

Before you start:
If you only have one Mac then print this article out first.* You’re not going to have access to your Mac and the Internet for a few hours (or more) as you follow through this article.

Are you using a MacBook or MacBook Pro?* Plug it into mains power.* It can really ruin your day to run out of power half way through formatting a hard drive or installing an operating system.

Part 1 - Prepare the Hard Drive

Most hard drives, when you buy them, are formatted to be used to hold data.* Except in a few cases, they’re likely to be formatted with Windows in mind.* While your Mac can also use these drives, you won’t be able to set one up to boot without some extra work.

WARNING: Depending on your drive configuration, you’re going to lose everything on this hard drive.* If there’s anything on it that you can’t afford to lose, now would be a really good time to back it up somewhere else.

1. Boot your Mac, wait for it to finish booting up normally.


2. Plug in your external hard drive and wait for the icon to appear on the desktop.


3. Launch Disk Utility (usually found in Applications -> Utilities).


4. Select (click on) your external hard drive.


5. Select your volume scheme (I like to use 2 partitions).


6. Give each partition a name (this helps you easily identify them later - I use “Boot” and “Data”).


7. Set format to “Mac OS Extended (Journaled) - there’s other options but this is my personal preference.








8. Click Options.


9. Make sure the partition scheme is set to “GUID Partition Table” and click OK.


10. Click Apply in Disk Utility to save all your changes.

That’s it for Disk Utility.* If you’ve followed this through right, your external hard drive should now be ready to be used as a boot drive for Snow Leopard.

Part 2 - Install Mac OS X

Now comes the fun part!

Time to install Mac OS X onto your external hard drive.

You should already have your Mac booted and have your external hard drive (configured in Part 1) plugged in and ready to use.

Put your install DVD in your drive and wait for the “Mac OS X Install DVD” window to come up.



When you see it, double click on “Install Mac OS X”.

Select “Continue” and “Agree” to the software license agreement.

Depending on what was already on your external hard drive, you may get the above window offering to install OS X on your internal (built-in on your computer) hard drive.* If you see this, click on “Show All Disks”.

Select the drive (or partition) you prepared in Part 1 above.* Normally, this will be an orange coloured drive and have a symbol on the top indicating the type of connection to your computer (USB in the screenshot above).* Once you’ve selected where you want to install Mac OS X, click “Install”.

WARNING:* Be sure you choose the right disk.* If you choose the internal hard drive in your Mac, the installer will happily go ahead and install over the top - this may result in you losing data or having to re-install applications.

At this point, Mac OS X will start installing on your external hard drive.* This might take quite a while if you’re using USB.

At some point in the process, your Mac will reboot automatically and then continue the install.

Don’t worry, this is normal.* It doesn’t mean there’s anything wrong and (as long as you got the above steps right) won’t affect your normal install of Snow Leopard.

If you’ve done everything right, the install will finish, your Mac will reboot and start up with the initial Welcome process for setting up a Mac for the first time.

Part 3 - Choose Your Boot

At this point, you’ve got an external hard drive that you can use to run Mac OS X from instead of the internal hard drive that’s built into your computer.

By default, your computer is going to want to boot from this hard drive next time you turn it on.

If you turn your computer off and unplug your external hard drive, though, your Mac will normally figure out that this drive isn’t available anymore and go back to booting from the internal drive in your computer.* Be warned, though, that if you do this, the next time you connect your external hard drive you’ll find yourself booting from it again.

There are two other good ways to change your boot drive.

1. As you first turn on your computer, press and hold down the Option key.* Just after the screen changes from black to grey, you’ll be shown which hard drives you can boot from.* Select which drive you want to boot from and then click on the up arrow to boot.* If your computer is turned off, this is the preferred option.
   


2. If your computer is already booted, you can set the boot drive by going into System Preferences and then selecting “Startup Disk”.Then you can select from the available options for booting your computer.* Select the drive you want and then click on “Restart...”.You’ll get a warning that you’re about to restart your computer, which, if you go ahead, will then restart your system.

Part 4 - Now What?

Everything up to here has been about getting your external hard drive ready to be used.* Now that you’ve got the drive prepared and you know how to boot from either your built-in internal hard drive or your newly prepared external hard drive it’s time to look at what you can do with it.

For starters, you’ll need a drive like this if you’re going to prepare a disk image for building a standard install of Mac OS X as detailed in this article.

You can install diagnostic testing tools on your external hard drive to check that your computer is working normally (some of these are already installed as part of Snow Leopard - e.g. Disk Utility) but you could also install TechTool Deluxe (included with Apple Care Protection Plan if you purchased this).

Are you planning to upgrade to a bigger hard drive in your Mac?

Booting from an external hard drive is one good way of helping this happen.* In fact, you could use a variation of this process to create a complete copy of your internal hard drive on an external hard drive as a “just in case” that could keep you working if something goes wrong with your built-in hard drive.

David Freeman is the proprietor of Outback Queensland Internet (aka Leading Edge Computers Longreach).* While a recent returnee to the world of Apple he has worked as a technician for over fifteen years and been involved in computers and the Internet since 1988 when he purchased his first computer (an Amiga 1000).