Install OS X on external drive

Purpose

To install, then run a full OS X installation from an external devices, such as a USB disk.

Details

At the time of this writing OS X (Maverick 10.9.5), is not distributed on physical medium anymore. Free, as in beer, upgrades are done via the App Store, and the installer takes care of migrating and upgrading the shiny box to an even more shiny desktop.

However, wether you want to experiment with the OS, prepare disk images to replicate and distribute in a portable way (ie, so as to populate pre configured OS X installs for large installations), or simply needs to upgrade your current disk starting with a clean install, you can use the OS X Installer to put the OS on virtually anything connected to your machine.

Step by Step Recipe

• Look for OS X Mavericks (or whatever that is current) in the App Store and install it. Alternatively you can get one of these unofficial 5+GB torrents that contains the dmg file of the Mavericks installer;
• Using Disk Utility, prepare the USB device so that it contains a GUID partition table, containing one partition, and formatted as Mac OS Extended (Journaled);
• Start the OS X Maverick installer;
• Your device, while not visible at first, can be selected when clicking on the advanced/more/options button;
• Proceed with the installation;
• When the machine reboots, it will automatically resume the installation process on the external disk;
• When asked about your Apple ID and registration, and unlike what is implied, you do not have to fill anything, you will still be able to benefit from the system software updates via the App Store;
• When the installation is finished, you will land in a pristine, so-called clean install, OS X, that might already have some software updates to apply immediately.

And then?

If all went well:

• If you turn on the machine without the USB drive, you boot into the original, default, OS X;
• If you turn on the machine with the USB drive connected, you boot into the OS X installed on the external disk.