How to create an emergency MacOsX Mavericks recovery disk on a bootable USB drive

Starting from Lion (Mac OS X 10.7), Apple operating systems no longer come with a bundled install DVD, which has been replaced by a hidden recovery partition that can be used for the same purpose. While this has surely meant a remarkable cost slashing for Apple, on the other hand it has created some inconveniences to users. It's actually pretty easy to reinstall Mac OS X from Recovery Partition, but it requires an active Internet connection in order to download the needed files from Apple servers. This is a consequence of the so-called cloud computing revolution. But, what if we need to reinstall Mac OS X without an active internet connection or we cannot access Wi-Fi from outside the operating system?

Luckily there's an easy fix for this problem: create an emergency boot disk on a bootable USB drive that will work the same way as an install DVD. 

LEGAL DISCLAIMER: The procedure outlined in this tutorial explains how to create a USB emergency recovery tool to boot lawfully purchased Mac computers running a genuine MacOsX version and isn't meant in any way to promote or facilitate the creation of a "Hackintosh". The author of the article reminds you that running Apple software on non-Apple hardware infringes the terms of service and is against the law. The author can't be held responsible under any point of view for a distorted use of the below instructions.

• Step 1:  Check requirements:  

You'll need an external USB flash drive, at least 8 GB big, and Mac OS X 10.6.8 (the last update released for Snow Leopard), or a later version, and the latest MacOsX Mavericks installation file.

• Step 2: Get the latest MacOsX Mavericks installation file: You can download it from AppStore or you can use an older installer if you have already one on your iMac. The former option is by all means recommended, because when Apple releases a new version of the installer, if you have an older file already present on your Mac, it becomes outdated and won't be replaced by a new file (if you have already installed Mavericks, you'll need to check App Store for system updates only).
  

Fig. 2 (Click to enlarge)

• Step 3: Prepare the USB drive:

Once you have the latest installation file, you'll need to format the USB drive you want to use by opening Disk Utilities (from Applications/Utilities). After choosing the destination disk and clicking Partition tab, you'll have to select Format and choose Mac OS extended (Journaled) as a file system for the drive to be recognized by Mac OS X. That done, choose Options from Disk Utility and then GUID Partition Table to make the USB drive bootable on a Mac. Afterwards click OK, Apply and Restore to format the disk.

Fig. 3 (Click to enlarge)

• Step 4: Prepare the needed files to be copied on the USB drive.

After downloading the Mavericks Installer, you'll find a file called Install OS X Mavericks.app in the Applications folder, that you'll use to create your bootable USB. Right-click it and select Show Package Contents from the next menu (Fig. 3). From the folder you'll be shown, open Contents and then Shared Support. You’ll find a disk image file called InstallESD.dmg. Double-click it and now there'll be a new volume in the Finder called OSX Install ESD.

Fig. 4 (Click to enlarge)

 

  

Step 5: Display hidden files to complete the preparation of the USB drive.

Now you need to work on another disk image (BaseSystem.dmg), contained in the volume you have just created (i.e. OSX Install ESD).  This file is invisible, which makes it necessary to enter a command in the Terminal to display all files and folders, including the invisible ones: defaults write com.apple.finder AppleShowAllFiles 1 && killall Finder.

After entering this command, you'll be able to access the needed file (BaseSystem.dmg) and youll need to double-click it to mount its disk image in the Finder, that'll be named  OS X Base System.

  

•  Prepare the USB drive

At this point you can create your bootable USB installer. Open Disk Utility and click Restore tab.
Drag BaseSystem.dmg from the Finder to the Source field and have your pendrive in the Destination field. Click Restore and then confirm by selecting Erase to start the process, which will require a while (Fig. 8).

 After the operation has been completed, eject OS X Base System by right-clicking it in the Disk Utility windows and choosing Eject. This is necessary because your USB pendrive will be given the same name and you would have two OS X Base System volumes, making things too confusing.

Now you need to access the System folder of your pendrive, open the Installation folder and delete the alias called Packages that you'll find in it.
That done, open OS X Install ESD volume, and drag the Packages folder into the Installation folder on the USB drive which will replace the alias you've just deleted.

Fig. 9

Eject the OS X Install ESD volume.
If you like, you can rename your bootable installer drive from OS X Base System to something more significant, such as OS X Mavericks Installer.

• Step 7:  Boot from the USB drive to check that all works good.

After all this, open the Terminal and enter defaults write com.apple.finder AppleShowAllFiles 0 && killall Finder to restore the default settings, not to display hidden files and folders.
At this point you'll only need to make sure all looks good and your pendrive actually allows you to install MacOsX Mavericks; you can check it by pressing Alt (⌥) key (a.k.a. Option key) at Mac boot-up. You'll be shown a list of available devices, among which you'll see your pendrive (Fig. 10). If you choose to boot from it, you'll be displayed, after a little while, MacOsX Mavericks installation wizard (Fig. 11).

Fig. 10

Fig. 11

You can now enjoy your working bootable Mavericks USB installer. Have fun!

11/8/2014 Update: The same steps work with Yosemite (OS X 10.10) as well.  Of course, you'll have to use Install Yosemite.app for this purpose.

Advice: This procedure doesn't create a Recovery partition if there's not already one on your Mac. In the latter case, have a look here for more information.