Open Terminal and type (or copy and paste) the following commands, one by one, pressing return after each to run it. Make sure each command finishes--in other words, you see a command prompt--before running the next command. Enter your admin-level account password when prompted.
sudo hdiutil attach /Applications/Install\ OS\ X\ Yosemite\ Beta.app/Contents/SharedSupport/InstallESD.dmg
sudo asr restore -source /Volumes/OS\ X\ Install\ ESD/BaseSystem.dmg -target /Volumes/Untitled -erase -format HFS+
(During this step, you'll be prompted to confirm that you want to erase the contents of Untitled. Type y and press Return.)
sudo rm /Volumes/OS\ X\ Base\ System/System/Installation/Packages
sudo cp -a /Volumes/OS\ X\ Install\ ESD/Packages /Volumes/OS\ X\ Base\ System/System/Installation/Packages
sudo cp -a /Volumes/OS\ X\ Install\ ESD/BaseSystem.chunklist /Volumes/OS\ X\ Base\ System
sudo cp -a /Volumes/OS\ X\ Install\ ESD/BaseSystem.dmg /Volumes/OS\ X\ Base\ System
hdiutil detach /Volumes/OS\ X\ Install\ ESD
You now have a bootable Yosemite-beta install drive. If you like, you can rename the drive from its default name of OS X Base System to something more descriptive, such as Yosemite Beta Installer.
Booting from the installer drive
Whichever of the above processes you've used, you can now boot any Yosemite-compatible Mac from the resulting drive: Just connect the drive to your Mac and either (if your Mac is already booted into OS X) choose the 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 your Mac is booted from your installer drive, you can, of course, install the OS, but you can also use any of the OS X installer's special recovery and restore features. Depending on how you made your installer drive, when you boot from that drive, you may even see the same OS X Utilities screen you get when you boot into OS X Recovery (recovery mode). However, unlike with recovery mode, your bootable installer includes the entire installer.
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