How to Make Your Own Apple OS X Lion Bootable USB Key

In response to my story on how to restore Apple OS X Lion via the Internet, a number of readers wrote in to voice concern about their newfound reliance on broadband Internet connectivity. And with good reason: Plenty of folks simply don’t have access to fast or reliable Internet connections. While Apple will make OS X Lion available on a USB key, it’ll run you a $40 premium ($69) and you’ll have to wait until next month to get it. But it doesn’t have to be that way. Building on the instructions posted on OS X Daily, I’ve walked through the process of retrieving my Lion download from the Mac App Store and making my own bootable USB key. You can, too; it’s easier than you think.

What you’ll need
• A Mac with Snow Leopard and the Mac App Store.
• Mac OS X Lion. (Check out PCMag's full OS X Lion Review)
• 8GB USB key (after formatting I only had about 6GB available for the disk image).

What you need to know
I took up this challenge after I had installed OS X Lion on my MacBook Pro. It’s worth noting that once you run the Lion installer from the Mac App Store, it magically disappears from your computer. You, can, however, retrieve it in one of two ways. If you hold down the "Option" key when you click on the "Purchased" tab of the Mac App Store you'll gain the option to re-download the Lion installer. If you have another Mac with the Mac App Store, you can also take advantage of Apple's generous registration policy (Apple allows you to register up to five computers with an iTunes account). I signed into my iTunes account via another MacBook’s Mac App Store (Store/ Sign In) to re-download Lion. As long as you don’t run the installer, you can locate it in your Applications folder.

Finding what you need in the Lion installer

In your Applications folder you’ll see a file titled “Install Mac OS X Lion.app.” This alone won’t suffice for a bootable USB key. Instead, you’ll need to have a look inside that application. Right-click (control-click) on the app and select “Show Package Contents.” From here, navigate to the “SharedSupport” folder inside “Contents.” You’ll see a Lion disk image titled “InstallESD.dmg.” Double-click to mount the image, “Mac OS X Install ESD,” on your desktop.

Making your USB key bootable
With your Lion disk image mounted on the desktop, plug in your USB key and open Disk Utility (I launched it from Spotlight). In the left-hand pane of Disk Utility you’ll see a handful of drives. Select the USB key and navigate to the “Erase” tab. From here you’ll want to format it as a Mac drive: I changed my USB key from a DOS format to “Mac OS Extended (Journaled)” and clicked “Erase.”

Leashing Lion to your USB key
Once your USB key has been formatted as a Mac disk—it only takes a moment—right-click (control-click) the USB key in the left-hand pane and select “Restore.” You’ll see a window with fields for “Source” and “Destination.” The source of your restoration is the disk image on your desktop (“Mac OS X Install ESD”); the destination is your USB key. Apple makes selecting these targets easy: Simply drag and drop them from the left-hand pane. Make sure that the “Erase Destination” box is checked (silly, I know, considering we just erased the drive) and click “Restore.” Transferring Lion to my USB key on a MacBook took about 15 minutes.

Loosing Lion from your USB Key
Booting from your USB key is easy. I plugged my Lion-equipped USB key into an iMac running Snow Leopard and restarted the machine. Before the Apple icon appears on the gray screen, hold down the “Option” key. This will reveal all of the bootable options on your machine; for my iMac this included Snow Leopard, a Windows Bootcamp partition, and my USB key. Select the USB key and run the installer. Be advised, you won’t entirely obviate Apple’s Internet demands. I still noticed a message reading, “Downloading additional components;” however, the message disappeared within a minute, so I can suspect those components were limited in size and supplemental to the base Lion installer.

The advantages of keying Lion
If you have an 8 GB USB key and you’re prepared to purchase Lion from the Mac App Store, I see little reason not to make a bootable Lion backup. Ideally, you’d do this before you installed Lion, but, as I’ve shown, if you’ve already installed Lion, so long as you can find another Mac that can access the Mac App Store, you can sign in with one of your five registrations and re-download the Lion installer. The process is straightforward and it will ensure that you have some sort of physical backup of your operating system in case you ever need to restore your Mac with limited Internet connectivity. As mentioned, I still needed Internet connectivity to download “additional components,” but that process was quick, and certainly a lighter lift than downloading the entire 4 GB package.

Once you’ve purchased Apple OS X Lion from the Mac App Store, it’s your right to install Lion how you wish. If Apple permits users to install their latest OS on up to five machines associated with your iTunes account, it shouldn’t matter if customers rely on Apple’s servers or their own personal USB storage. Considering the simplicity of the process, there’s no need to wait—and pay a premium—for something you can do on your own today. If you have the extra USB key, give it a shot. It could take some of the bite out of upgrading your other desktops.