The machine will start to boot from the USB drive.

Once selected, either hit the Return key or double-click your selection.

Using either the pointer or arrow keys on the keyboard, select the USB drive you wish to boot from.

The utility will look for any available drives that include bootable content. Once the Startup Manager screen appears, release the Option key. Holding that key gives you access to OS X’s Startup Manager.

When you hear the startup chime, press and hold the Option key.

Press the Power button to turn on your Mac (or Restart your Mac if it’s already on).

Insert the USB boot media into an open USB slot.

Getting your Mac to load from a USB drive is fairly straightforward.

So if your system is unstable, you need to run a diagnostic tool on the hard drive, or you just want to load a Linux desktop just to see what it’s all about, let’s look at how you start your machine using rescue USB boot media. How you start that rescue media can vary depending on the operating system you are using, but there are a few general guidelines that can help get your machine started, regardless of the OS you prefer. As a result, booting from USB media is becoming the standard. And while users historically would pop a boot media disk into their DVD or CD drive, many computers no longer come with optical disk drives.

Check if usb is bootable