How to Install Mountain Lion

Hands on with Mountain Lion's OS X Recovery and Internet Recovery

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When Safari first launches in recovery mode, you’ll see a page called Recovery Information that provides brief instructions on how to perform various tasks in recovery mode. Unfortunately, you won’t be able to print these instructions from within recovery mode, although you can access them when booted from your standard startup drive—see “Recovery HD under the hood,” below.

To get back to the main OS X Utilities window, just quit Safari.

Disk Utility: Repair or erase a disk using Disk Utility. Selecting this option and clicking Continue launches Disk Utility, which you can use to check, repair, erase, or partition connected drives, including your startup drive. (Yes, you can partition your normal startup drive from within recovery mode, although be aware that doing so will erase the drive’s contents.)

If you decide to erase your Mac’s startup drive and reinstall OS X (after making sure you’ve backed up, of course), you’d start here, erase the drive, quit Disk Utility, and then use the Reinstall option (above) to install a new copy of OS X. You can get back to the Utilities window at any time by quitting Disk Utility.

Other options In addition to the four main functions in the Utilities window, recovery mode also offers a few options in its Utilities menu (displayed when viewing the main OS X Utilities screen): Firmware Password Utility, Network Utility, and Terminal. These are the same utilities you can use when your Mac is booted normally into OS X; they let you configure a firmware password, monitor network connections and traffic, and use OS X’s Unix shell, respectively.

What version of OS X does recovery mode install?

You may be wondering what version of OS X will be installed if you use recovery mode to reinstall the operating system. When Lion Recovery debuted last year, the answer to this question was simply, “Lion.” But now that there are two versions of OS X that support recovery mode, the answer is a bit more complicated.

Standard (non-Internet) OS X Recovery Unless there was a problem when installing OS X, your Recovery HD partition should correspond to your hard drive’s OS: Mountain Lion or Lion. (When you install Mountain Lion over Lion, Lion Recovery is updated to the Mountain Lion version.) In other words, if you’re running Mountain Lion and you reinstall OS X from within recovery mode, you get Mountain Lion. If you’re running Lion, you get Lion.

Internet Recovery Unlike OS X Recovery, Internet Recovery relies on your Mac’s firmware, which isn’t updated when you install a newer version of the OS. This means that if your Mac model was released prior to Mountain Lion, you have the Lion version of Internet Recovery, even if you upgrade your Mac’s operating system to Mountain Lion. If you purchase a Mac model released after Mountain Lion’s debut, Internet Recovery will install Mountain Lion. (It’s possible that Apple could release firmware updates for older Macs to give them the Mountain Lion version of Internet Recovery, but it hasn’t happened yet.)

What all this means is that as long as your Mac’s drive doesn’t have serious problems—so it can boot from the Recovery HD partition—holding down Command+R lets you re-install the same version of OS X as is currently installed on your hard drive. If your drive is having serious problems, so your Mac has to boot into Internet Recovery, reinstalling OS X gives you whatever version of the OS your Mac originally shipped with. (On Macs that shipped with Snow Leopard but gained OS X Internet Recovery support via a firmware update, Internet Recovery installs Lion.)

Recovery HD under the hood

If you try to find the Recovery HD partition in the Finder, or even using Disk Utility, you’ll come up empty. Apple’s hidden this partition well, presumably to keep it safe from accidental (or intentional) modifications—after all, what good is an emergency disk if someone has accidentally deleted some of its vital contents?

However, if your curiosity won’t be sated until you’ve been able to browse Recovery HD, here’s how. Just remember: Look, but don’t touch.

1) Open Terminal, type diskutil list, and press Return.

2) You’ll get output similar to this (If you’ve enabled FileVault, you may see a second list for /dev/disk1):

DanBookAir:~ frakes$ diskutil list
  #:                       TYPE NAME                    SIZE       IDENTIFIER
  0:      GUID_partition_scheme                        *121.3 GB   disk0
  1:                        EFI                         209.7 MB   disk0s1
  2:                  Apple_HFS DanBookAir              120.5 GB   disk0s2
  3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3

3) Locate the Recovery HD partition (under Name) and note its identifier—in my case, disk0s3.

4) Type diskutil mount [identifier], where [identifier] is, of course, that identifier. This mounts the Recovery HD partition in the Finder. Inside you’ll find a folder named

5) Open the folder in the Finder, and you'll see several items. However, you aren't seeing everything—some of the folder's contents are invisible. If you want to see everything that's there, switch back to Terminal and type (or copy from here and paste into Terminal) ls -al /Volumes/Recovery\ HD/ and press Return. This will display the full list of the folder's contents:

DanBookAir:~ frakes$ ls -al /Volumes/Recovery\ HD/
total 906680
drwxr-xr-x   9 root  wheel        306 Jul 12 23:12 .
drwxrwxr-x  11 root  wheel        442 Jul 13 00:56 ..
-rw-r--r--@  1 root  admin       1840 Jul  5 05:19 BaseSystem.chunklist
-rw-r--r--@  1 root  admin  448702992 Jul  5 04:58 BaseSystem.dmg
-rw-r--r--   1 root  wheel       3418 Jun 20 12:53 PlatformSupport.plist
-r--r--r--   1 root  wheel        475 Jul  5 02:56 SystemVersion.plist
-rw-r--r--   1 root  wheel     986272 Jul  5 04:57 boot.efi
-rw-r--r--   1 root  wheel        361 Jul 12 23:12
-rw-r--r--   1 root  wheel   14510577 Jul  5 04:53 kernelcache

Of particular interest is BaseSystem.dmg, a disk image that contains the recovery partition’s bootable copy of OS X and all the recovery-mode utilities. You can mount this disk image in the Finder by typing open /Volumes/Recovery\ HD/ and pressing Return. Once you’ve done that, you can view the Recovery Information page you see when you launch Safari from within recovery mode; just type open /Volumes/Mac\ OS\ X\ Base\ System/System/Installation/CDIS/OS\ X\ (all one line) and press Return. You’ll see the contents of the English.lproj folder; find the file RecoveryInformation.html and double-click it to open it in your default Web browser.

When you're done browsing, you can eject Mac OS X Base System as you would any removable volume. You can then unmount the Recovery HD volume by typing diskutil unmount [identifier], where [identifier] is the same identifier you used above.

[Dan Frakes is a Macworld senior editor. He wishes he had a recovery mode right about now.]

Updated 7/25/12, 7:55pm to mention the Command+Option+R shortcut for Internet Recovery and clarify the paragraph on booting into Internet Recovery.

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