Boot Osx From External Drive

By Michael Argentini
Managing Partner, Technology and Design

How do you erase your Mac hard drive and install OS X Yosemite without a startup disk? Normally the answer would be to boot into recovery mode. But what if you need to partition the hard drive, wiping the recovery partition? You need a bootable OS X Yosemite installer. Here's how to make one using Apple's built-in boot drive creation tool.

Use the flash drive (with clover installed) to boot the OS X hard drive. Install Clover onto the OS X hard drive, but instead of installing directly to the boot0af in MBR, select the options “Install for UEFI booting only” and “Install Clover in the ESP“. Also make sure the “RC Scripts” are installed. Mount the EFI partition of the. This option on boot trick works for quite literally any boot volume, whether it’s an external USB drive of any sort, a Thunderbolt hard drive, boot DVD, CD, the Recovery partition, even in dual-boot environments with other versions of OS X, or a Linux or a Windows partition with Boot Camp, if it’s bootable and connected to the Mac it will be visible at this boot manager.

  • Here are the steps to boot Mac from the USB flash drive: Power on the system. Press and hold the Option (Alt) key on the keyboard when the computer starts. Select the USB drive as a startup disk when the option appears. The system will start the boot process off the USB drive.
  • Change External Boot settings Use this feature to control whether your Mac can start up from an external hard drive, thumb drive, or other external media. The default and most secure setting is ”Disallow booting from external media.”.

What You Need

  1. The 5GB OS X Yosemite installer app that downloads into your Applications folder when you first install it via the Mac App Store. The file name is Install OS X Yosemite. After you upgrade to Yosemite, this file is deleted. So you have to quit the installer once it appears in order to keep this file.
  2. A flash drive, external hard disk, SD card, or the like, with at least 8GB of capacity.
  3. Your Mac user account must be an Administrator. If it isn't, go to Users & Groups in System Preferences and assign Administrator to your user account. You can change it back to a standard user when you're finished. You can also sign in to your Mac as the administrator to create the bootable drive.

Step 1: Format Your Drive

Boot Os X From External Drive Linux

First, you must partition and erase the drive so that it can be bootable. To do this, launch Disk Utility and choose the destination drive in the drive list (left column). On the right, in the Erase tab, choose to format the drive as Mac OS Extended (Journaled). These are the defaults, so just confirm the settings.

Next go to the Partition tab and under Partition Layout choose '1 Partition' in the dropdown menu. Change the name from 'Untitled 1' to 'Untitled'. This is so that the Terminal command below will find the drive. Once the process is complete, the drive name will be 'Install OS X Yosemite'.

Click the 'Options' button. In the dialog that appears, choose 'GUID Partition Table' as the partition scheme, then click 'OK'. Again, this is the default, so just confirm the setting.

Finally, click 'Apply' to confirm the settings and prepare the drive.

Step 2: Make the Drive a Yosemite Boot Installer

In a single command you'll create the bootable install drive. Pretty sweet! To do this, launch Terminal and enter the following command. Note, this assumes that you haven't moved the installer out of your Applications folder. If you have, simply replace '/Applications' with the location of the installer:

The sudo command will ask for your password. As you type it will not appear. This is normal. Just enter it and press return to create the boot drive.

During installation, you'll see a progression of status messages.

Wait until you see a status of 'Done.' When you do, you can safely eject the drive and quit Terminal.

Step 3: Start Up from the Install Drive

If you open the new bootable drive in the Finder, you'll notice that it appears empty, other than the Install OS X Yosemite app. No worries. The boot files are hidden, and the installer app allows you to upgrade without booting from the drive if you don't need to.

To boot from this new installer, shut down the Mac, plug in the bootable drive, and hold down the option key while powering back up. You should eventually see a series of bootable drives. Click on the one named 'Install OS X Yosemite' and press the return key.


Where to next?

You can run macOS on a Mac in a variety of ways that we have already written an informative article about. In this article, we will focus on Mac startup mode with booting from a USB or an external hard drive.

Starting a Mac with booting from an external disk may be necessary if you need to run another version or copy of macOS, other OS, fix any problems, and so on.

To start the following conditions must be met:

  • The computer is based on the Intel system;
  • The volume is formatted with the choice of GUID section type;
  • Mac OS X v10.4.5 or later is installed on the USB storage device.

Read more:How to Use a External Hard Drive for Time Machine Backups?

How to Boot Mac from an external Hard Drive or USB device?

  • Connect a USB flash drive or external hard drive to your Mac.
  • Turn on your Mac by pressing the power button or restart your computer if it is already running;
  • Hold down the horkey for boots options: Option (Alt) key on the keyboard and hold until the boot menu appears;
  • Select the desired volume using the mouse, arrow or trackpad;
  • Press the Enter key to boot the Mac from the selected volume.


If Mac won’t boot from USB or volume is not displayed, wait a few seconds for the Download Manager to finish scanning the connected disks or use next tips, if it’s useless here the list of troubleshooting.

macOS does not Boot from an External Drive, what Should I Do?

Boot Os X From External Drive Usb

  • Some older external USB drives require additional power. It is possible that it must be connected to an external power source or use a second USB on a Mac;
  • Make sure the external drive is turned on (again, the prerogative of old USB-drives);
  • Restore the disk access rights and correct the errors on it;
  • Make sure the disk is formatted with the choice of partition type GUID;
  • Try connecting an external drive to another USB port;
  • Make sure the external drive is bootable;
  • Connect the drive directly, without using a USB hub.