How To Boot Mac From External Hard Drive


How to boot from the installer drive. With the external drive plugged in: Intel Mac: Restart/power up the Mac and hold down the option key on the keyboard during the boot process. To make a Mac act as an external hard disk, you can put it in Target Disk Mode and connect it to another Mac with a FireWire or Thunderbolt cable, as follows. Note: Target Disk Mode is available on most recent Macs; however, computers that do not have FireWire or Thunderbolt ports are excluded.

  • Nov 01, 2021 Boot from the External Hard Drive. Now that macOS Monterey is installed on your external hard drive, it’s time to tell your Mac to boot from it. Luckily, macOS makes this simple: Launch System Preferences. Go to Startup Disk. Unlock the preference pane by clicking the lock icon. Select your external hard drive.
  • Oct 30, 2021 1 Figure out your computer’s BIOS key on Windows. Step 1: Launch the search engine. Be it any search engine e.g. Step 2: In the search box, type the computer’s manufacturer, the name of the model, and the phrase bios key. Click enter to continue. Step 3: Related to your search, the browser.

In this article, I’m going to show you how to run macOS directly from an external hard drive. Not only does this let you switch between operating systems, but it’s also a great way to experience different versions of macOS without permanently changing anything on your Mac. For example, if your favourite app only runs on a certain version of macOS, or you want to test your project across multiple versions, then an external hard drive can come in handy!

By the end of this article, you’ll know how to run any version of macOS, directly from your external hard drive.

Prepare your external hard drive

In order to install macOS, you’ll need an external drive with at least 12GB of free space.

Once you have a suitable hard drive, the first step is preparing this drive as a GUID partition:

  • Connect the external drive to your Mac.
  • Open a new Finder window and navigate to ‘Applications > Utilities.’
  • Launch the Disk Utility app.
  • In the upper-left corner of the Disk Utility app, select the ‘View’ button and make sure ‘Show All Devices’ is selected.
  • In Disk Utility’s left-hand menu, select your root drive.
  • Click the ‘Erase’ button.
  • In the subsequent window, open the ‘Format’ dropdown and choose ‘Mac OS Extended (Journaled).’
  • Open the ‘Scheme’ dropdown and choose ‘GUID Partition Map.’
  • Give your drive a name.
  • Select ‘Erase.’
  • Disk Utility will now create the partition and setup the drive, which may take a few minutes. Once the process is complete, click ‘Done.’

Download any version of macOS

The next step, is downloading the installer for the version of macOS that you want to run from your external hard drive.

If you want to install a version of macOS that you previously downloaded from the App Store, then you can just re-download the necessary installer:

  • Launch the App Store.
  • Select the ‘Purchases’ tab.
  • Find the version of macOS that you want to install, and then click its accompanying ‘Install’ button. This will download the installer to your Mac’s ‘Applications’ folder.

If you want to run a version of macOS that you haven’t previously downloaded from the App Store, then things are going to get slightly trickier, as you won’t be able to download this version directly from the App Store.

The easiest workaround, is to try and find another Mac user who’s willing to download the installer for you, as macOS installers aren’t user specific.

Install macOS on your external hard drive

Once you have the necessary files, you’re ready to install this version of macOS to your external hard drive:

  • Open a new Finder window.
  • Navigate to the installer that you want to use.
  • Click to launch the installer.
  • Click ‘Continue.’
  • Read the terms and conditions, and if you’re happy to proceed, then click ‘Agree,’ followed by ‘Continue.’
  • Click ‘Show All Disks,’ and then select your external drive.
  • Click ‘Install.’

This version of macOS will now be installed on your external hard drive.

Boot into your new operating system

To boot into this version of macOS:

  • Make sure the external drive is attached to your Mac.
  • Hold down your Mac’s ‘Option’ key during startup.
  • When prompted, select your external drive.

Your Mac will now boot into the version of macOS that’s stored on your external drive. To switch back to your original version of macOS, simply repeat the above steps.

Before you go

After spending over 20 years working with Macs, both old and new, theres a tool I think would be useful to every Mac owner who is experiencing performance issues.

CleanMyMac is highest rated all-round cleaning app for the Mac, it can quickly diagnose and solve a whole plethora of common (but sometimes tedious to fix) issues at the click of a button. It also just happens to resolve many of the issues covered in the speed up section of this site, so Download CleanMyMac to get your Mac back up to speed today.

6 6 likes 86,364 views Last modified Nov 4, 2013 8:26 AM

Hello and welcome to my User Tip

This is a step by step instruction how to make and boot a bootable clone of your OS X system.

It can be used for backup, moving to a larger drive, moving your users to a new Mac (using Migration Assistant), defragmenting and optimizing the system (with reverse clone), shifting data up on hard drives to make more room for BootCamp or another partition (reverse clone) or as a data recovery/undelete boot drive.

Made before disaster strikes, it's a real lifesaver!

See these other related User Tips.

Cloning is a bootable backup system, it doesn't fix issues in software

If you need to make a new/clean bootable system on a external drive to reverse clone with

How cloning/reverse cloning using CCC can defrag and optimize your boot hard drive (SSD's no effect)

Read this for the differences, advantages and disadvantages of TimeMacine and other backup systems.

Problems with not enough space for a BootCamp partition and how cloning/reverse cloning can resolve that

Getting the right external drive

Get a external hard drive equal to or larger than the drive you intend to replace the internal one with. (if so) So later it will be bootable backup (clone) of that new drive.

If you like the idea of saving the differences between the clone updates, then get a much larger drive. If maintaining a pure clone, then the same size or slightly larger than your primary boot drive will do.

Get a self powered and not a Mac port powered drive! Why? Because Mac's can cut off external hardware drawing too much power which can cause issues trying to boot from the clone.

If you have Filevault enabled on your boot drive, then this User Tip has no further use for you, sorry. To de-Filevault your drive will likley require 50% or more free space available to write all the unencrypted bits. Once the drive is de-Filevaulted it can be cloned.


Connect the new drive to the Mac and open Disk Utility in the Utilities folder. (say 'no' to TimeMachine, use another drive for that). Click on the external drive on the left and click Erase, (10. 6 users Secure erase Zero all Data) or move the slider for a 0x-3x secure erase (SSD's not needed), OS X Journaled/GUID as the format and a name like 'Mac10.6Clone' (different than the internal drive) and click apply. Go watch a movie. Warning: Formatting a drive or partition erases all data.

If the scrubbing of the drive fails, try again and if it fails, the drive is bad and needs to be exchanged. All new drives have bad sectors, this scrubbing will remove most of them and make for a much more reliable backup. Highly recommended for all hard drives if it hasn't been done before, SSD's no need.

Once the scrubbing is finished check under Partition that Option: GUID and Format: OS X Extended Journaled or make it that. Quit Disk Utility.

Scrubbing is only for hard drives and not needed if it was done once before as the bad bits were previously mapped off. However if shock to the drive occurred or the 0x erase wasn't good enough, or there is unwanted data, then of course repeat scrubbing again. I've found 0x for defeating software recovery techniques, 3x seems to be adequate for mapping off the bad bits. 7x for defeating magnetic recovery techniques. Physically destroying the drive for the utmost sensitive files.

SSD's cannot be securely erased, however it might be possible to overwrite one's preious deleted files with new data with a method I've outlined here.

Cloning software

Download Carbon Copy Cloner, it's the best cloning software and the only one that also clones the most vital RecoveryHD partition for restoring OS X fresh from Apple. Later you will use it to update your clone once your on the new internal drive. (Optionally is SuperDuper, but it doesn't do the RecoveryHD.)

Use CCC to clone your present internal drive to the external. Do not use the same name as the internal drive on the external, or change it before you boot from the clone using the Finder.

You don't need to mess with CCC preferences for the first clone as it has to to everything. Later when updating the clone it will save the changes between updates and take up much more drive space. If you want to maintain a pure clone, then set that in CCC preferences before doing a update, this is what I advise as it maintains space for later less you need to install additional software or move files for data recovery etc. while booted from the clone drive if the internal drive fails to boot up.

Repair permissions afterwards

Once the clone is finished, use Disk Utility to repair permissions on both drives. Reboot the Mac and hold the option/alt keys down on the built-in or wired keyboard for OS X's 'Startup Manager' there you can select the clone to boot from. Check it out and be careful with your navigation as everything is duplicated, including the pathnames/shortcuts to your internal drive. (Ignore Disk Utility warnings that repeat, those are just changes Apple made)

Internal drive switch

If you have a internal drive switch or install at this time, now is the time to do this. If you run into problems you can option/alt key boot off the clone drive, use the computer like before and get online. TimeMachine doesn't have boot to use ability only boot to restore, which mandates your internal drive be immediately fixed and that's not always very convenient for many.

More advice here:

Create Bootable External Hd

Reverse cloning, etc

To reverse clone, follow the same procedure as above. First formatting the new or erasing (secure zero or 3x overwrite) the corrupted drive (if so) in Disk Utility first and then using CCC (all while option/alt key booted from the external clone of course), to reverse clone. CCC should ask to restore the RecoveryHD (for 10.7+) in this first clone process, so check that it will/does.

Bootable clones make ideal data recovery drives, just boot from it and grab the latest copy of files (or install Data Rescue for deleted files/corrupted drives) on the primary boot drive and secure erase and reverse clone perfection back on.

It's advised to maintain several time dated clones each on separate hardware, this way one can revert to a earlier OS X version or use older versions of software and to protect against accidents and other unseen acts. Provides maximum software and hardware protection.

Do NOT reverse clone onto a new or different model of Mac as there are hardware driver differences so OS X is ever so slightly different. (But in some cases it's possible, not going there). Instead to be sure, use Migration Assistant in the Utilities folder and target the clone drive as the source of User Accounts and programs you want to transfer to the other Mac.

How To Boot Mac From External Hard Drive For Xbox One

Corrupted cache rebuilding (optional)

How To Boot Mac From External Hard Drive

See and perform the #12 OnyX cache cleaning routine here, it's optional but if your reverse cloning it's best to also rebuild the cache files so they are free of corruption and maximize the full performance benefit of the reverse clone proceedure.

Remember 10.8+ users may have to right or option/alt click to 'open' software downloaded from the Internet.

SSD advice, TRIM

If you cloned to a third party SSD, you need to enable TRIM support on it and after each OS X update/upgrade using third party software. Check for the two versions and update it. Apple doesn't reenable TRIM for third party SSD's only their own.

Remember 10.8+ users may have to right or option/alt click to 'open' software downloaded from the Internet.

TRIM is wear leveling software so that data is written to the least used areas of a SSD because they have limited write capability. Thus SSD's don't have scrubbing ability to securely delete data off of them. Software and forensic equipment is widely available to read all data on a SSD, included deleted data.

SSD's are on many portable tech electronic devices and becoming widespread on computers now. The NSA approved method for data destruction on non-magnetic media (including thumb drives) is to grind them into a fine powder. Since a lot of Apple hardware is coming sealed up, Fusion drives (hard drive + flash memory), and non-user accessible, the entire device will have to be destroyed in order to remove unwanted data from it.

For privacy when reselling without vital data, it might be possible to fill the SSD near completely two times with a LOT of tiny small files to overwrite all areas, but that's no guaranty nor a legal/military/government approved method.

Update your clone

Update your clone occasionally and before a major internal boot drive change using CCC, it will take less time than a full clone each time like Disk Utility unfortunately does. CCC also has scheduling ability to perform the updates automatically.

Do not clone a TimeMachine drive.

Clone to another internal partition

Yes you can clone your boot partition to another partition on the same boot drive. Clone to a external drive first and test for backup as you NEED hardware protection also in case the internal drive or Mac dies or is lost/stolen.

Create another partition in Disk Utility equal to your MacintoshHD partition, format OS X Extended Journaled and use CCC to clone MacintoshHD to MacintoshHD 2. Then Disk Utility >Repair Permissions on both. (ignore warnings that repeat, those are just changes Apple made) Set CCC to maintain a pure clone for this partition as not to fill it up.

If you can't create another same sized partition, you likely need to reduce files and (if a hard drive, SSD's no need) perform a reverse clone while booted from the external clone to shift all OS X data up further on the drive to make room at the bottom for the new partition.

Usefulness in this clone on another partition thing is CCC can update it in the background on a schedule all by itself. You have a resource to recover accidentally deleted files and if your primary OS X MacintoshHD partition is not booting due to software reasons or other software issue, you can immediately option/alt key boot from the cloned partition. A excellent choice for laptop owners away from a fast/reliable Internet connection to fix OS X by redownloading it over itself or carrying a external clone or TM drive around with you. It only takes seconds to boot from the cloned partition.

How To Boot Mac From External Hard Drive

It's obviously taking half your boot drive space, however it only takes Disk Utility a few seconds to rename/quick reformat (no secure erasing) to make it available for emergency storage needs and later one can use CCC to clone it again in the background. Also on hard drives the second 50% of the drive is slower than the first 50%, so you may realize this while booted from the second partition.

How to boot mac from external hard drive best buy

How To Boot Mac From External Hard Drive Usb

Boot Macbook From Usb

How To Boot Mac From External Hard Drive Instructions

Enjoy peace and software stability. 🙂