How do I boot from my external drive on a Mac?

Clone it with a utility like Carbon Copy Cloner and then hold down the Option key while booting to choose the external drive.

Macs are a little finicky that way. Unlike a PC, you cannot set the boot order to tell it to look at external drives first, so you pretty much have to hold the Option key when you boot up.

It is possible that it would do it automatically if the internal drive had no valid operating system to boot from, but I am not 100% sure that OS X does the same thing that every other OS (including older versions of the Mac’s OS) does.

@jerv- It does boot from it if it can’t find the internal one. It just needs a GUID Partition Table when the drive is formatted.

Other then that crisw nailed it. And you can also go into System Preferences and set the external as the start-up disk. That way you can skip the step of holding down option when you boot. And also keep in mind that booting from the external will be a lot slower.

@world_hello

Good point. I had an external 10.6 boot drive on a USB key to use in imaging some of our Macs and it took about 20 minutes to boot up…

You would actually have the best performance if you booted off the internal drive and stored all your files on the external. You can also move all users’ home directories to the external if you go to Settings > Accounts, click the lock, right click on a user, go to Advanced Options, and change the home directory.

@Vortico Great suggestion

If you’re going to have an external there all the time- you might as well boot off your internal drive- it will be faster- and store your data on the external.

It’s actually a pretty smart setup to boot off an internal drive not cluttered with all your data. Make sure your external is a fast drive though with a fast connection since you will be accessing it constantly- and probably one with a good well ventilated enclosure that is quiet. In this situation it’s probably worth it to pay more for a really nice external drive- and store it well- I’ve had externals hooked up semi-permanently and accidentally knocked them off the desk. (And destroyed them!)

The only other downside is since the computer has multiple users will be teaching your brothers as much about the setup as you know. Like make sure they know not to unmount the external etc- or that if they want to go root around in their home directory to pick that drive not the internal.