Disk Utility Can Create a Bootable OS X Yosemite Installer

Make a bootable version of OS X Yosemite from a flash drive

OS X Yosemite (10.10) can be downloaded for free from the Mac App Store. Once downloaded, the installer will start up automatically. If you follow the onscreen instructions, you'll end up with an upgrade install of OS X (or macOS) Yosemite on your startup drive.

What if you want to perform a clean install, completely erasing your startup drive? Or perhaps you'd like to have the installer on a bootable USB drive, so you don’t have to keep downloading it every time you wish to upgrade one of your Macs?

Illustration of a USB stick with label OS X Yosemite
Lifewire / Alex Diaz Dos

The problem is you cannot upgrade another Mac without downloading the installer all over again. The workaround is to create a bootable USB flash drive containing the OS X Yosemite installer.

While we prefer using a USB flash drive as the destination for the installer, you can use any bootable media, including hard drives, SSDs, and USB flash drives.

Use Disk Utility to Create a Bootable OS X Yosemite Installer

USB flash drive with laptop
bluehill75 / Getty Images

There are two ways to create a bootable installer.

The first is to use a hidden Terminal command that can handle all of the heavy lifting for you.

The second is more manual and time-intensive, requiring the use of the Finder and Disk Utility apps. This article will take you through the steps to manually create a bootable copy of the OS X Yosemite installer.

What You Need

  • OS X Yosemite installer. You can download the installer from the Mac App Store. Once complete, you'll find the download in the /Applications/ folder, with the file name Install OS X Yosemite
  • A USB flash drive or another suitable bootable device. As mentioned, you can use a hard drive or an SSD for the bootable device, although these instructions will refer to a USB flash drive.
  • A Mac that meets the minimum requirements for OS X Yosemite.

The process for creating a bootable copy of the OS X Yosemite installer follows these basic steps:

  1. Mount the installer on your desktop.
  2. Use Disk Utility to make a clone of the installer.
  3. Modify the clone to allow it to boot successfully.

Mount the OS X Yosemite Installer

OS X Yosemite Installer's Package Content

Deep within the Install, OS X Yosemite Beta file that you downloaded is a disk image that contains all of the files you need to create your own bootable installer. The first step is to gain access to this image file.

  1. Open a Finder window and navigate to /Applications/.

  2. Locate the file named Install OS X Yosemite, then right-click the file and select Show Package Contents.

  3. Open the Contents folder, then open the Shared Support folder.

  4. Here you will find the disk image that contains the files we need to create a bootable installer. Double-click the InstallESD.dmg file.

    This will mount the InstallESD image on your Mac desktop and open a Finder window displaying the contents of the mounted file.

  5. You may notice that the mounted image seems to contain only a single folder, named Packages. In actuality, there's an entire bootable system on the image file that is hidden. We need to use Terminal to make the system files visible.

    Follow this link for instructions on how to show hidden files in Finder:

  6. With the files now visible, you can see that the OS X Install ESD image contains three additional files: .DS_Store, BaseSystem.chunklist, and BaseSystem.dmg. We're going to use this Finder window in subsequent steps, so leave this window open.

With all the files we need now visible, we can move on to using Disk Utility to create a clone of the OS X Install ESD image that we mounted on the desktop.

Use Disk Utility to Clone the OS X Install ESD Image

Using Disk Utility to Create a Clone of OS X Yosemite Installer

The next step is to use Disk Utility's Restore feature to create a clone of the OS X Install ESD image you mounted on your desktop.

  1. Launch Disk Utility, located at /Applications/Utilities/.

    Make sure the target USB flash drive is connected to your Mac.

  2. Select the BaseSystem.dmg item listed in the left-hand pane of the Disk Utility window. It may be listed near the bottom, after your Mac's internal and external drives. If the BaseSystem.dmg item is not present in the Disk Utility sidebar, you can drag it into the sidebar from the Finder window that appeared when you mounted the InstallESD.dmg file.

    Make sure you select BaseSystem.dmg, not InstallESD.dmg, which will also appear in the list.

  3. Select Restore.

  4. In the Restore tab, you will see BaseSystem.dmg listed in the Source field. If not, drag the BaseSystem.dmg item from the left-hand pane to the Source field.

  5. Drag the USB flash drive from the left-hand pane to the Destination field.

    The next step will completely erase the contents of the USB flash drive, or whichever bootable device you dragged to the Destination field.

  6. Select Restore.

  7. You will be asked to confirm that you want to erase the USB flash drive and replace its contents with BaseSystem.dmg. Select Erase.

  8. If requested, supply your administrative password and select OK.

  9. The restore process will take some time. Once it's complete, the Flash drive will mount on your desktop and open in a Finder window named OS X Base System. Keep this Finder window open, because we'll be making use of it in subsequent steps.

We're done with Disk Utility, so you can quit this app. All that's left to do is modify the OS X Base System (the flash drive) to make the OS X Yosemite installer work correctly from a bootable device.

Modify the OS X Base System on Your Flash Drive

Copying Packages Folder in Mac OSX Yosemite

So far, we found the hidden image file within the Yosemite installer. We created a clone of the hidden image file, and now we're ready to copy a couple of files that will make the bootable version of the OS X Yosemite installer work correctly.

We're going to be working in Finder, with the two windows we asked you to keep open in the first set of instruction. It can get a bit confusing, so read through the following steps first, to make sure you understand the process.

  1. In the Finder window named OS X Base System, open the System folder, then open the Installation folder.

  2. Within the Installation folder, you will find an alias named Packages. Delete the Packages alias by dragging it to the trash, or by right-clicking the alias and selecting Move to Trash from the pop-up menu.

    Leave the Installation window open, because we'll use it below.

  3. Open the Finder window named OS X Install ESD. (If you didn't leave this window open from earlier steps, follow the instructions in Step 2 to re-open the window.)

  4. From the OS X Install ESD window, drag the Packages folder to the Installation window you left open in the previous step.

  5. From the OS X Install ESD window, drag the BaseSystem.chunklist and BaseSystem.dmg files to the OS X Base System window (the root level of the USB flash drive) to copy them to the flash drive.

  6. Once the copying is complete, you can close all of the Finder windows.

Your USB flash drive is now ready to be used as a bootable OS X Yosemite installer.

You can boot from the Yosemite installer you just made by inserting the USB flash drive into your Mac, then starting your Mac while holding down the option key. This will present the Apple boot manager, which will let you select the device you wish to start up from.

Important: Make Files Invisible Again

Earlier, we made invisible files and folders visible. It's time to return those items to their original invisible state. Follow these instructions to hide those files: