As with the Mavericks installer, if you leave the Yosemite beta installer in its default location (in the main Applications folder) when you install OS X 10.10, the installer will delete itself after the installation finishes. If you plan to use that installer on other Macs, or--in this case--to create a bootable drive, be sure to copy the installer to another drive, or at least move it out of the Applications folder, before you install. If you don't, you'll have to redownload the installer from the Mac App Store before you can create a bootable installer drive.
Create the Yosemite beta install drive: The options
I've come up with three ways you can create a bootable OS X install drive for the Yosemite beta: using the installer's built-in createinstallmedia tool; using Disk Utility; or performing the Disk Utility procedure using Terminal.
The createinstallmedia method is the easiest; if you're at all comfortable using Terminal, it's the approach that I recommend you try first. (Note that the createinstallmedia tool doesn't work under OS X 10.6 Snow Leopard--it requires OS X 10.7 Lion or later.)
The Disk Utility method is the way to go for people who are more comfortable in the Finder (though it does require a couple Terminal commands), and it works under Snow Leopard, Lion, Mountain Lion, Mavericks, and Yosemite. The Disk Utility-via-Terminal approach is for the shell junkies out there.
Whichever method you use, you need a Mac-formatted drive (a hard drive, solid-state drive [SSD], thumb drive, or USB stick) that's big enough to hold the installer and all its data--I recommend at least an 8GB flash drive. That drive must also be formatted with a GUID Partition Table. (Follow this tutorial to properly format the drive.) Your OS X user account must also have administrator privileges.
Note that the current version of the Yosemite beta installer does not install a Recovery HD partition.
Option 1: Use createinstallmedia
Starting with Mavericks, hidden inside the OS X installer is a Unix program called createinstallmedia, provided by Apple specifically for creating a bootable installer drive. If you're comfortable using Terminal, createinstallmedia is a relatively simple tool to use.
As mentioned above, the createinstallmedia tool works only in Lion, Mountain Lion, Mavericks, or Yosemite--you can't create an installer drive this way while booted into Snow Leopard. If you need to create a Yosemite beta install drive while booted into Snow Leopard, you should use the Disk Utility instructions, below.
Here are the required steps:
Connect to your Mac a properly formatted 8GB (or larger) drive, and rename the drive Untitled. (The Terminal command used here assumes the drive is named Untitled.) Also, make sure the Yosemite beta installer, called Install OS X Yosemite Beta.app, is in its default location in your main Applications folder (/Applications).
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