I stole this from Macworld:
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.
Using the createinstallmedia command in Terminal
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 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.
- 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
- Launch Terminal (in /Applications/Utilities).
- 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.
- Type your admin-level account password when prompted, and then press Return.
- 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.