How to Boot OS X From External Hard Drive

By K.C. Winslow

Intel-based Mac computers are able to boot up from an external USB hard drive that include a version of OS X 10.4.5 or later. To install OS X on an external drive, you must have a drive formatted in the GUID partition scheme. The partition scheme determines how the computer divides a drive into sections, even if there is only one partition. Booting up from an external hard drive is beneficial when you need to perform troubleshooting on your start-up disk, which is the hard drive your computer normally uses to boot up.

Things You'll Need

  • Mac OS X Install disc, 10.4.5 or later

Step 1

Connect the external drive to the computer.

Step 2

Open the “Go” menu in the Finder and select “Utilities.” Double-click the “Disk Utility” icon.

Step 3

Click on the icon for the volume of the external drive in the pane on the left side of the window. The volume appears indented beneath the drive.

Step 4

Click the “Info” icon at the top of the window. Look for the partition map scheme. If the scheme listed is “GUID,” skip to Step 7. If you see another scheme, you will need to repartition the drive. This will erase the contents of the drive, so back up important files before proceeding. Click the red button in the upper, left corner of the “Info” window to close it.

Step 5

Click the “Partition” tab at the top of the Disk Utility window. Press the “Options” button and select the GUID partition scheme. Click “OK.”

Step 6

Enter the number of partitions you would like on the drive, if you want to have more than one. Click “Partition” to erase the drive and format it in the GUID scheme.

Step 7

Load the OS X Install disc in the drive of the computer. Double-click on the “Install” icon. A prompt to restart the computer appears. Press the “Restart” button and the computer boots up from the installation disk.

Step 8

Follow the on-screen instructions to install OS X on the external disk. Restart the computer once the installation is complete.

Step 9

Hold down the “Option” key on the keyboard to bring up the “Startup Manager.” Select the external drive to boot up using the drive.