Like Lion (OS X 10.7) before it, Mountain Lion (OS X 10.8) doesn’t ship on a disc - it’s available only as an installer app downloadable from the Mac App Store, and that installer doesn’t require a bootable installation disc. But there are a good number of reasons you might want a bootable Mountain Lion installer on an external hard drive or USB stick.

For example, if you want to install Mountain Lion on multiple Macs, a bootable install drive can be more convenient than downloading or copying the entire Mountain Lion installer to each computer. Also, if your Mac is experiencing problems, a bootable install drive makes a handy emergency disk. (Mountain Lion’s OS X Recovery feature, known as Lion Recovery prior to Mountain Lion’s release, is a big help here, but not all Macs get it - and if your Mac’s drive is itself having trouble, recovery mode may not even be available. Also, if you need to reinstall Mountain Lion, recovery mode requires you to download the entire 4GB+ installer again.) Finally, if you need to install Mountain Lion over Leopard - assuming you have the license to do so - a bootable install drive makes it easier to do so.

Thankfully, it’s easy to create a bootable install drive from the Mountain Lion installer that you download from the Mac App Store. I show you how, below.

You may have noticed that I didn’t mention making a bootable install DVD. Though it’s possible to make one, I don’t recommend it these days. More and more Macs ship without a built-in optical drive; booting and installing from a DVD is very slow; and 8GB flash drives can be found for $10 or less—there’s little reason to opt for a DVD anymore. In addition, you can easily update a USB stick or external hard drive each time an update to Mac OS X is released, as explained below; with a DVD, you have to toss the disc in the trash and start over, which is both a hassle and bad for the environment.

Get the latest version of the Mountain Lion installer

Before you make a bootable install drive, you should make sure you have the latest version of the Mountain Lion installer. What? You didn’t even realise there are different versions of the installer? It turns out that when you download the Mountain Lion installer from the Mac App Store, that copy of the installer installs whatever version of OS X was available at the time of download. For example, if you downloaded OS X 10.8 the day Mountain Lion is released, you downloaded the 10.8.0 installer. Which means that a bootable install drive you create from that installer will install OS X 10.8.0.

However, unlike with the CD- and DVD-based Mac OS X installers of old, which can never be updated once they’re created, Apple regularly updates the Mountain Lion installer on the Mac App Store so that it installs the latest version of OS X 10.8. For example, when the inevitable 10.8.1 update is released, a few days later the Mac App Store will begin providing an updated Mountain Lion installer that installs 10.8.1 right off the bat. Using an updated installer is convenient, because it means that if you ever need to re-install Mountain Lion, you won’t have to install 10.8.0 and then immediately install the latest big update.

Obviously, then, you want to create your bootable install drive using the latest version of the Mountain Lion installer. However, unlike with other Mac App Store-purchased software, the Mac App Store does not update the copy of the Mountain Lion installer app on your hard drive. If you’ve got an older version of the installer and you want the latest version, you must re-download Mountain Lion from the Mac App Store. (If the Mac App Store won’t let you re-download the installer, quit the Mac App Store app, relaunch it, and then Option+click the Purchases tab in the toolbar; that should show the Download button next to Mountain Lion in the Purchases list.)

Similarly, any bootable Mountain Lion install drive you create will not automatically be updated to the latest installer version. If you create an install drive and later download an updated version of the Mountain Lion installer, updating your install drive means erasing it and repeating the procedure below.

How do you know if you have the newest version of the Mountain Lion installer? There’s a file inside the installer that indicates which version of OS X it will install, but getting to that file and viewing it is messy. The easiest approach is to simply look at the Information box on the Mountain Lion page on the Mac App Store - specifically, check the date next to Updated. Then locate your downloaded copy of the Mountain Lion installer in the Finder, choose File -> Get Info, and look at the date next to Modified. If the Mac App Store date is newer than the Modified date on your copy of the installer, you need to re-download the installer to get the latest version. (The version listed in the Mac App Store’s Information box is the version of OS X you’ll get if you download the installer.)

A note on installer compatibility

The initial Mac App Store version of Mountain Lion will boot only those Macs released prior to Mountain Lion’s debut—Macs released after Mountain Lion’s debut will ship with a newer version of Mountain Lion preinstalled. This means that if you make a bootable install drive right when Mountain Lion debuts, and then later buy a new Mac, your install drive won’t boot that Mac. However, as explained above, Apple regularly updates the Mountain Lion installer on the Mac App Store so that it installs the latest version of OS X 10.8. If you create a new bootable installer using the first major update to Mountain Lion after your Mac was released, it should be able to boot all your Macs.

There’s a catch here, however: Recent Macs are designed to let you re-install the OS using Internet Recovery. So if you buy a new Mac post-Mountain Lion, and you haven’t purchased Mountain Lion for another Mac, you can’t download the Mountain Lion installer from the Mac App Store. For Lion, I explained how to create a bootable install drive for newer Macs. Once Apple starts shipping Macs with a Mountain Lion version of Internet Recovery, I’ll update that article to cover Mountain Lion.

Create the Mountain Lion install drive

There are a couple ways you can create a bootable install drive: using OS X’s own Disk Utility or using the third-party utility Carbon Copy Cloner. The latter is easier, but the former doesn’t require you to download third-party software. (The developer of Lion DiskMaker will be releasing, soon after Mountain Lion’s debut, an update to that utility that supports Mountain Lion, giving you yet another option.)

Note that whichever method you use, you need a hard drive or thumb drive (USB stick) at least 5GB in size (an 8GB flash drive works well), and it must be formatted with a GUID Partition Table. Follow Steps 1 through 4 in this slideshow to properly format the drive.

Using Disk Utility 

You’ll find this utility in your Utilities folder (in /Applications/Utilities). Here are the steps for using it to create your installer drive:

  • Once you’ve purchased Mountain Lion, find the installer on your Mac. It’s called Install OS X Mountain and it should have been downloaded to your main Applications folder (/Applications).

  • Right-click (or Control+click) the installer, and choose Show Package Contents from the resulting contextual menu.

  • In the folder that appears, open Contents, then open Shared Support; you’ll see a disk image file called InstallESD.dmg.

  • Launch Disk Utility.

  • Drag the InstallESD.dmg disk image into the bottom (empty area) of Disk Utility’s sidebar (on the left).

  • In Disk Utility, select InstallESD.dmg in the sidebar, and then click the Open button in the toolbar to mount the disk image’s volume in the Finder. The mounted volume is called Mac OS X Install ESD, and it also appears below InstallESD.dmg in Disk Utility.

  • Select Mac OS X Install ESD in Disk Utility’s sidebar, then click the Restore button in the main part of the window.

  • Drag the Mac OS X Install ESD icon into the Source field on the right (if it isn’t already there).

  • Connect to your Mac the properly formatted hard drive or flash drive you want to use for your bootable Mountain Lion installer.

  • In Disk Utility, find this destination drive in the left-hand sidebar and then drag it into the Destination field on the right. (If the destination drive has multiple partitions, just drag the partition you want to use as your bootable installer volume.) Warning: The next step will erase the destination drive or partition, so make sure it doesn’t contain any valuable data.

  • Click Restore, and then Erase in the dialog box that appears; if prompted, enter an admin-level username and password.

  • The restore procedure will take roughly five to ten minutes, depending on your Mac and the speed of your drive(s).

(If you ever created a bootable Lion-install drive, you may have performed the above procedure without mounting the InstallESD disk image. As of version 10.7.4 of the Lion installer, and continuing with the Mountain Lion installer, you must use the mounted Mac OS X Install ESD volume or you’ll get an error at the end of the restore procedure, and the newly created bootable drive may not function properly.)

Using Carbon Copy Cloner 

Version 3.5 or later of this excellent clone-backup utility includes a special feature for creating a bootable install drive. (Carbon Copy Cloner is free to try; you can purchase a license for $40.) Once you’ve downloaded it from the developer’s website - or, if you already had it, once you’ve checked to make sure you’ve got the latest version - follow these steps:

  • Find the Mountain Lion installer on your Mac. It’s called Install OS X Mountain and it should have been downloaded to your main Applications folder (/Applications). Note that Carbon Copy Cloner requires that the installer be located in the Applications folder on your boot drive, which means that if you followed my advice to move the installer out of your Applications folder, you’ll have to move it back, at least temporarily.

  • Connect to your Mac the properly formatted hard drive or flash drive you want to use for your bootable Mountain Lion installer.

  • Launch Carbon Copy Cloner.

  • In the Source pop-up menu (labeled Select A Source), choose Create a Mountain Lion Installer. (This mounts the InstallESD.dmg disk image, mentioned above, and selects it as the source volume.)

  • In the Destination pop-up menu (labeled Select A Destination), choose the drive you’re using for your bootable installer.

  • .In the Handling Of Data Already On The Destination pop-up menu, choose Delete Anything That Doesn’t Exist On The Source. Warning: The next step will effectively erase the destination drive or partition, so make sure it doesn’t contain any valuable data.

  • Click Clone, click Continue in the warning dialog that appears, and, when prompted, provide an administrator username and password.

As with using Disk Utility, the process takes roughly five to ten minutes. One difference here: Whereas using Disk Utility names your bootable drive Mac OS X Install ESD, using Carbon Copy Cloner keeps the drive’s original name.

Booting from the installer drive

You can now boot any Mountain Lion-compatible Mac from this drive: Just connect the drive to your Mac and either (if your Mac is already booted into OS X) choose your install drive in the Startup Disk pane of System Preferences or (if your Mac is currently shut down) hold down the Option key at startup and choose the install drive when OS X’s Startup Manager appears.

When booted from your install drive, you can, of course, install the OS, but you can also use any of the Mountain Lion installer’s special recovery and restore features - in fact, when you boot from this drive, you’ll see the same OS X Utilities screen you get when you boot into restore mode. However, unlike restore mode, your bootable installer includes the entire installer.