How to Boot a Mac into an External Drive or Alternate Boot Disk Guide

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Certain situations require you to start your Mac from an external startup disk instead of the primary startup disk. There are many reasons to boot from external disks, whether you are troubleshooting problems, repairing disks, partitioning, formatting everything, updating everything, or even reinstalling Mac OS X. We discuss both ways it is easiest to do this during startup in Mac Boot Manager , and also through System Preferences through the Startup Disk Selection tool.

You need some kind of bootable drive to do this, whether it’s a USB installation drive of your own, macOS Mojave, macOS High Sierra, macOS Sierra, Mac OS X El Capitan, Yosemite, OS X Mavericks, or earlier versions of Mac OS X , a mirrored external hard drive made through SuperDuper, or even a boot disk. All Macs with Lion (10.7) or later can use Startup Manager when booting from the recovery partition.

Boot from an external device when you restart with Mac Boot Manager

This can be used to select any startup drive associated with your Mac:

  • Connect an external drive or device to your Mac
  • Restart your Mac and after booting press and hold OPTION during startup until you see the startup option
  • Click external volume to turn it on

You will find that external drives are usually shown with an orange icon and their interface is printed on the icon itself. CDs and DVDs are indicated, respectively, by a disc icon. In this screenshot example, the correct orange boot drive is a USB flash drive.

Boot your Mac from the external drive

This boot system option works on literally every boot disk, whether it’s an external USB drive, Thunderbolt hard drive, bootable DVD, CD, recovery partition, even in dual-boot environments with other versions of OS X, or a Linux or Windows partition with Boot Camp , if it is bootable and connected to your Mac, it will appear in this Startup Manager.

Although the bootable DVDs and CDs are visible through the boot manager mentioned above, you can boot your Mac directly to an attached DVD or disc by holding down the D key during restart when you hear a beep. This is very rare these days, but it was the main way to access recovery partitions before Mac OS X became downloadable from the App Store and before USB installation drives became more common.

Additionally, Macs with recovery partitions can be booted directly into Recovery HD by holding down Command + R during system startup.

Although it is possible to recover and start disks with their own commands, the Option key method is ultimately easier to remember because it is a single key and because it is universal. The only exception is the target disk space, which requires using a different sequence.

Also note the option to join a Wi-Fi network in Startup Manager, which can be left alone if you don’t need Mac OS X Internet Recovery.

Choose a different startup drive from Startup Disk Manager

You can also choose a different startup volume from the Startup Disk in System Preferences:

  • Connect the bootable drive to your Mac
  • Open System Preferences  from the Apple menu and select “Startup Disk”
  • Choose a new mapped startup drive from the list, and then click Restart or restart your Mac normally to start from this disk.

Click Startup Disk in Mac OS X System Preferences

One thing to consider with this approach is that the option remains in effect until it is changed again using Startup Disk Manager. This means that if the selected startup disk is removed from the Mac, a flashing question mark may appear on the next reboot because the startup disk is no longer visible. If this flashing question mark is persistent, you can easily correct it by holding down the OPTION key, selecting the normal Macintosh HD boot size at boot time as described above, and then the correct MacOS or Mac OS boot size Select X again to return to the System Preferences Boot Disk.

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