How do I install OSX Yosemite on USB?

Here are the required steps:

  1. 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 installer, called Install OS X Yosemite.app, is in its default location in your main Applications folder (/Applications). This means that if you moved it before installing Yosemite, you need to move it back before making your installer disk.
  2. Select the text of this Terminal command and copy it:  
    sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
  3. Launch Terminal (in /Applications/Utilities).
  4. Warning: This step will erase the destination drive or partition, so make sure that it doesn’t contain any valuable data. Paste the copied command into Terminal and press Return.
  5. Type your admin-level account password when prompted, and then press Return.
  6. The Terminal window displays the progress of the process, in a very Terminal sort of way, by displaying a textual representation of a progress bar: Erasing Disk: 0%... 10 percent...20 percent... and so on. The program then tells you it’s copying the installer files, making the disk bootable, and copying boot files. Wait until you see the text Copy Complete. Done. (see the screenshot above), which could take as long as 20 or 30 minutes, depending on how fast your Mac can copy data to your destination drive.

You now have a bootable Yosemite install drive. If you like, you can rename the drive from its default name of Install OS X Yosemite, though I think it’s kind of a catchy name.

Option 2: Use Disk Utility

You’ll find Disk Utility, a handy app that ships with OS X, in /Applications/Utilities. Here are the steps for using it to create your installer drive. The procedure is a bit more involved with Yosemite than it was for Mavericks (which was itself a bit more involved than under Mountain Lion and Lion).

yosemite beta show package contentsRight-click (or Control+click) the Yosemite installer to view its contents.
  1. Once you’ve downloaded Yosemite, find the installer on your Mac. It’s called Install OS X Yosemite.app and it should have been downloaded to your main Applications folder (/Applications).
  2. Right-click (or Control+click) the installer, and choose Show Package Contents from the resulting contextual menu.
  3. In the folder that appears, open Contents, then open Shared Support; you’ll see a disk image file called InstallESD.dmg.
  4. Double-click InstallESD.dmg in the Finder to mount its volume. That volume will appear in the Finder as OS X Install ESD; open it to view its contents.
  5. Several of the files you’ll need to work with are hidden in the Finder, and you need to make them visible. Open the Terminal app (in /Application/Utilities), then type (or copy and paste) the following command, and then press Return: 
    defaults write com.apple.finder AppleShowAllFiles 1 && killall Finder
    (This tells the Finder to show hidden files—we’ll re-hide such files later.)
  6. Launch Disk Utility (in /Applications/Utilities) and then drag BaseSystem.dmg (in the OS X Install ESD volume) into Disk Utility’s left-hand sidebar.
  7. Select BaseSystem.dmg in Disk Utility’s sidebar, and then click the Restore button in the main part of the window.
  8. Drag the BaseSystem.dmg icon into the Source field on the right (if it isn’t already there).
  9. Connect to your Mac the properly formatted hard drive or flash drive you want to use for your bootable Yosemite installer.
  10. In Disk Utility, find this destination drive in the left sidebar. You may see a couple partitions under the drive: one named EFI and another with the name you see for the drive in the Finder. Drag the latter—the one with the drive name—into the Destination field on the right. (If the destination drive has additional partitions, just drag the partition you want to use as your bootable installer volume.)
  11. Warning: This step will erase the destination drive or partition, so make sure that it doesn’t contain any valuable data. Click Restore, and then click Erase in the dialog box that appears; if prompted, enter an admin-level username and password.
  12. Wait for the restore procedure to finish, which should take just a few minutes.
  13. Open the destination drive—the one you’re using for your bootable installer drive, which has been renamed OS X Base System. Inside that drive, open the System folder, and then open the Installation folder. You’ll see an alias called Packages. Delete that alias.
  14. Open the mounted OS X Install ESD volume, and you’ll see a folder called Packages. Drag that folder into the Installation folder on your destination drive. (You're replacing the deleted Packages alias with this Packages folder.) The folder is about 4.6GB in size, so the copy will take a bit of time, especially if you’re copying to a slow thumb drive.
  15. Also in the mounted OS X Install ESD volume, you’ll find files named BaseSystem.chunklist and BaseSystem.dmg. Copy these files to the root(top) level of your install drive (OS X Base System, not into the System or Installation folder).
  16. Eject the OS X Install ESD volume.
  17. You’ll likely want to re-hide invisible files in the Finder. Open the Terminal app, type (or copy and paste) the following command, and then press Return: 
    defaults write com.apple.finder AppleShowAllFiles 0 && killall Finder

You now have a bootable Yosemite install drive. If you like, you can rename the drive from OS X Base System to something more descriptive, such as Yosemite Installer.

yosemite beta disk utility restoreYou can use Disk Utility's Restore screen to create a bootable Yosemite installer drive.

Option 3: Use Terminal

If you're a Terminal jockey, you likely know that most of Disk Utility's features can be accessed using shell commands—which means that you can perform the Disk Utility procedure using a few commands in Terminal.

  1. Download the Yosemite installer from the Mac App Store and make sure it’s in your main Applications folder (/Applications)—it’s called Install OS X Yosemite.app.
  2. Connect to your Mac a properly formatted 8GB (or larger) drive. Rename the drive to Untitled. (The Terminal commands I provide here assume the drive is named Untitled.)
  3. 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.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 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 Installer.

yosemite beta terminal procedureYou can perform the Disk Utility procedure entirely in Terminal.

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.

Thanks to http://www.macworld.com/article/2367748/how-to-make-a-bootable-os-x-10-10-yosemite-install-drive.html for an awesome guide!

 
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