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.
How to Boot a Mac from an External Drive or USB?


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?

  • 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.

It’s the best time to ask questions and give us your feedback in comments.


Hi there! My Name is Vincent Lago! My goal is to share insightful reviews, guides, and manuals for people looking to know more about current solutions for Apple hardware and OS. I have years of technological background knowledge including owning a custom iMacs & PCs which was using for different types of business. And now, I want to share my experience with you.

1 Comment

  1. I realise this is an old post, but it seems relevant to an issue I’m experiencing. I have an old iMac Pro (early 2008) and in order to try and speed it up as it has a maximum 4gb memory, I decided to try an SSD drive. Rather than fitting it internally, it is an external USB drive. I successfully run Ubuntu 20.10 on the Mac on an external SSD dual booting using rEFInd so I wasn’t expecting problems.

    However as the Mac only has 3 USB ports I started off connecting a good, powered USB 3.0 hub to one port, the Mac metal keyboard to the second and the external SSD drive to the powered hub. But it won’t boot (or even see) this drive. If I connect it direct to the Mac I have no problem.

    So this left me with two SSD drives on two ports and the keyboard on the USB hub, but in this configuration, the Option (or any) of the startup keys are not recognised. But once the login screen appears, the keyboard is fully active.

    This means if I need to use the keyboard at power up, I have to unplug the hub to plug the keyboard in, and then once I’ve passed this point, unplug the keyboard so I can plug the Hub in, and plug the keyboard back into the hub.

    I know the keyboard has two USB ports, but some of my devices and memory sticks are too chunky to go under the keyboard, thus the need for the hub.

    Am I missing something as surely this can’t be right.

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