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

Click the external volume to boot from it.

Reboot the Mac and after the startup chime hold down the OPTION key during boot until you see the boot selection menu.

Connect the external drive or device to the Mac.

This can be used to select any bootable drive connected to the Mac: How to Boot from an External Device on Restart with Mac Boot Manager Any Mac running Lion (10.7) or newer will also be able to use the Boot Manager to startup from the Recovery partition. 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 macOS Mojave, macOS High Sierra, macOS Sierra, Mac OS X El Capitan, Yosemite, for OS X Mavericks or prior versions of Mac OS X, a mirrored external hard drive made through SuperDuper, or even a boot disc. 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.

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 Mac OS X. Certain situations require a Mac to be booted from an external boot volume, rather than the primary startup disk.

Os x utilities no startup disk