Use the following steps you can easily set mac boot from an external drive in startup manager so it ll only boot from usb that one time. With the mac powered off connect the external bootable drive to the thunderbolt 3 port.
To do this open disk utility and connect the drive to your mac.
Boot mac from external drive. The file format has to be mac os x journaled and you have to use the guid partition map. If you have multiple usb drives connected then the startup manager shall list the drives with the bootable os. For powerpc based macs the external drive must be attached via firewire while intel based macs can boot from either firewire or usb external hard drives.
This way you can boot your mac with an external usb drive but the most critical part is to prepare the usb drive. In this article we will focus on mac startup mode with booting from a usb or an external hard drive. 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.
Select your external hard drive. Under external in the left hand menu click on your external hard drive and then click on the erase button. Turn on your mac or restart your mac if it s already on.
Find your external hard drive s name or classification e g external hard drive in the boot order list then press the down arrow key until the external hard drive is selected. To start the following conditions must be met. Turn the mac back on with a long press of the power button holding until the screen shows startup options.
At this point booting from an external drive is the same as it always is. You can also change the startup disk from within system preferences in macos. So this is how to boot mac from usb drive.
In some cases you ll only be able to select usb devices or similar in this menu. Make sure you backup any data before you erase the drive. To boot from a connected external volume connect the boot drive if you haven t done so already then hold down the option key during system restart and select it during system start.
If the drive requires an external power source make sure the power is connected and that the drive turned on. Common problems and fixes when boot mac from external drive. If so make sure that your external hard drive is the only usb storage attached to your computer when booting from it.
Insert the usb boot media into a usb slot. Step 1 connect the external drive to the computer you wish to boot. 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.
Finally press return key on the selected option to boot mac. The computer is based on the intel system.