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