How to Boot a Mac from an External Drive or Alternate Startup Disk

Certain situations require a Mac to be booted from an external boot volume, rather than the primary startup disk. There are plenty of reasons for booting from external volumes, whether it’s to troubleshoot problems, repair disks, partition, format everything, update, or even reinstall OS X. We’ll cover the two easiest ways to do this, during boot with the Mac Boot Manager, and also through System Preferences using the Startup Disk selection tool.

You’ll need a bootable drive of some sort to be able to do this, whether it’s a USB installer drive you made yourself for OS X Mavericks or prior versions of Mac OS X, a mirrored external hard drive made through SuperDuper, or even a boot disc. Any Mac running Lion (10.7) or newer will also be able to use the Boot Manager to startup from the Recovery partition.

Booting from an External Device on Restart with Boot Manager

This can be used to select any bootable drive connected to the Mac:

• Connect the external drive or device to the Mac
• Reboot the Mac and after the startup chime hold down the OPTION key during boot until you see the boot selection menu
• Click the external volume to boot from it

You will find that external drives typically are shown with an orange icon, with their interface printed on the icon itself. Similarly, CD’s and DVD’s are shown with a disc icon. In this screen shot example, the right-most orange boot drive is a USB flash disk.

This 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.

Though boot DVD’s and CD’s will be visible through the aforementioned boot manager, you can also start the Mac directly to DVD or a connected disc by holding down the “D” key during restart after you hear the chime. This is fairly uncommon these days, but it was the primary method of accessing recovery partitions before OS X became a download from the App Store, and before USB installer drives became more common.

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

Though recovery and discs can be booted with their own commands, it’s ultimately easier to just remember the Option key method since it is a single key and because it’s universal. The only exception is with target disk mode, which requires a different sequence to use.

You’ll also notice an option to join a wi-fi network when at the Boot Manager, that can be left alone unless you need to perform an internet restore of OS X.

Selecting a Different Boot Drive from Startup Disk Manager

You can also select a different boot volume from the Startup Disk system preference panel:

• Attach the boot drive to the Mac
• Open System Preferences from the  Apple menu and choose “Startup Disk”
• Select the newly connected boot volume from the list, then choose “Restart” or reboot the Mac as normal to start from that disk
  
  

One thing to consider about this approach is that the setting choice stays in place until it has been changed again through the Startup Disk Manager. This means that if the chosen startup volume has been disconnected from the Mac, on the next reboot a blinking question mark may appear, because the set startup disk is no longer visible. If that blinking question mark is persistent, that can be easily remedied by holding down the OPTION key, selecting the normal Macintosh HD boot volume at startup as outlined above, and then going back to Startup Disk in System Preferences to choose the proper OS X startup volume again.