For the purposes of this walkthrough, we’re going to use a 16GB USB flash thumb drive, but you’re free to use whatever you want, and you could even use an external hard drive if you really wanted to. The convenience of a portable flash drive is undeniable so it’s preferred if you’re going to be updating multiple Macs. Let’s get started:
This is going to format the drive so that it will be a bootable installer, without doing this the drive may not be bootable. If you don’t want to erase the drive, find one you don’t mind formatting instead. When you format a fresh drive on a Mac it becomes labeled as “Untitled” so we’re going to assume that the boot destination drive is named Untitled as well. If for some reason you have another drive named Untitled, change the name or don’t use it.
Now that the drive is ready, you can move on to making the actual installer.
The next step will actually make the installer drive from the previously formatted USB disk. If you already have the OS X Yosemite installer application in the /Applications/ folder on the Mac, you can skip directly to step 3:
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
"Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
Making disk bootable...
Copying boot files...
Sidenote: rarely, you may encounter a “can’t be verified” error, here’s how you can fix that.
That’s all there is to it. Your freshly made OS X Yosemite install drive will be visible in the Mac Finder:
Now you just need to boot from the freshly made OS X Yosemite drive, do that by rebooting the Mac and holding down the OPTION key and selecting the “Install OS X Yosemite” drive upon boot.
If the drive does not boot, you almost certainly skipped the first step which was to partition the drive as GUID, or perhaps interfered with the syntax in the command. You can go through the process again to be sure.
This allows you to update any version of OS X from Snow Leopard, Lion, Mountain Lion, or Mavericks, directly to OS X Yosemite, with the installer that was just made. You can also perform a clean install (which we separately address in a thorough walkthrough here), or update multiple Macs without having to download it from the App Store again. Be sure to always back up the target Mac before updating OS X, whether to OS X 10.10 or any other version, you can follow detailed instructions on how to prepare a Mac for OS X Yosemite here. Enjoy OS X Yosemite!
The aforementioned steps have been tested repeatedly and are confirmed to work flawlessly with the OS X Yosemite final release. If you have any issues, run through the steps again, or leave a comment with your specific error. If you know of an easier way, let us know in the comments too!