How to Boot From a USB Drive on a Mac

By Julius Vandersteen

When you turn on your Mac, the OS X operating system loads from the startup disk, which typically is the primary internal hard drive. You can also install the Mac OS X operating system on an external USB drive to start up a Mac running 10.4.5 or later. You might want to be able to boot from a USB drive, such as when there is a problem with the Mac's internal hard drive, or when you want to run a diagnostic test on it.

Things You'll Need

  • Mac OS X installation disk


Step 1

Connect the USB drive to a USB port on your Mac with a USB cable. The drive will mount on the Mac's desktop.

Step 2

Click "Applications" in the Dock, then click "Utilities" and then select "Disk Utility." Click on the icon of the USB drive in the left side of the Disk Utility window.

Step 3

Click "File" and then click "Get Info." Look at the description next to "Partition Map Scheme." If it says "GUID Partition Table," that means you can use this USB drive as currently formatted to install the Mac OS X operating system. Otherwise, you will need to repartition the drive before you can install OS X on it. Back up any files that you want to keep before you repartition the USB drive, a procedure that erases and reformats the disk.

Step 4

Click the red circle at the top of the information window to close it.

Step 5

Click the "Partition" tab if you need to repartition the USB drive. Click "Options" and then choose "GUID" for the partition scheme, and then click "OK." Click "Partition."

Step 6

Insert a Mac OS X installation disk into the Mac's optical drive. Use an installation disk with an OS equal to or later than the version running on the Mac's startup drive. Click on the USB drive you want to install OS X onto, and then click "Install."

Boot from the USB Drive

Step 1

Click the Apple icon from the menu at the top of the screen, and then click "System Preferences."

Step 2

Click "Startup Disk."

Step 3

Click on the USB drive that has OS X installed on it.

Step 4

Click "Restart." The Mac will now boot up from the USB drive.