The traditional method of creating a bootable OS X USB installer no longer works, so you’ll need to follow a new approach when dealing with OS X Yosemite. While there are multiple methods that will work, here’s the easiest way to create a bootable OS X 10.10 Yosemite USB Installer for the Yosemite Public Beta.
Note: As mentioned, these steps are for the free OS X Yosemite Public Beta. If you’re a registered developer using the Developer Preview, follow these similar, but specialized, instructions.
Step 1: Obtain the Yosemite Public Beta Installer
If you’re registered for the Yosemite beta program, download the OS X Yosemite Public Beta installer from the Mac App Store. This will put a file called Install OS X Yosemite Beta.app in your /Applications folder. It will also launch the Yosemite installer app when the download completes. Quit the app by pressing Command+Q.
Step 2: Format and Prepare USB Drive
Grab a USB flash drive that’s at least 8GB in size. While it’s possible to create a separate partition for the Yosemite USB installer, it’s safest and easiest to use an empty drive or one that you don’t mind erasing. Plug the drive into a Mac running OS X 10.7 Lion or higher and launch Disk Utility from the /Applications/Utilities folder.
In Disk Utility, select the USB flash drive from the list on the left. Note that you want to select the drive and not the volume. We’re using a SanDisk Cruzer flash drive, so in our case we choose 8 GB SanDisk Cruzer Media and not the default “No Name” volume.
With the USB drive selected, choose the Partition tab on the right side of the window. The partition scheme and volume properties of your flash drive will vary depending on manufacturer and previous configuration. In our case, our drive is brand new and formatted as a FAT volume with a Master Boot Record partition scheme. This won’t work for Yosemite, so we need to change it.
In the drop-down menu under Partition Layout, choose 1 to create a single new partition. Then click Options, choose GUID Partition Table, and click OK to save the change. Under Partition Information, change Format to Mac OS Extended (Journaled) and give the drive the name “Untitled” (this allows your drive to work with the Terminal commands below; you can rename the drive when the process is complete).
Press Apply to restructure the USB volume with the new parameters. Note that this will erase all contents of the USB drive so, as mentioned above, be sure to back up any files on the drive or use a blank drive to begin with.
Step 3: Create the Bootable Yosemite USB Installer with Terminal
Now that your USB flash drive is ready, we can complete the process of creating a bootable Yosemite USB installer with a simple Terminal command. Open Terminal from /Applications/Utilities and then enter the following command:
sudo /Applications/Install\ OS\ X\ Yosemite\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite\ Beta.app --nointeraction
Press the Return key on your keyboard to execute the command, and enter your admin password when requested. This will create a bootable Yosemite USB installer using OS X’s createinstallmedia tool, which can take quite a while depending on the speed of your flash drive.
Let the tool do it’s thing and don’t interrupt the process until you see Terminal output Done and return your window to the user prompt. When it’s complete, your USB installer will be mounted to your Desktop and you can now rename this drive (highlight it on the Desktop and press Return), as well as supply it with its own custom icon.
Eject your new Yosemite USB installer and connect it to any Mac you wish to upgrade to the Yosemite Public Beta. Reboot the Mac holding the Alt/Option key on the keyboard and you’ll see the installer appear in the EFI boot menu. Select it and follow the prompts to install OS X Yosemite.
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