How to Boot a Mac With an External Hard Drive

This keyboard shows the Option and Command keys used in external booting.

To start up a Mac from a bootable external hard drive, hold down the proper key combination while the computer is starting up. The exact key combinations that can be used vary slightly from one type of Mac to another. Also, you cannot boot from just any external hard drive. To be bootable, the external drive must be partitioned properly and contain an installation of the proper Mac operating system (OS). For PowerPC-based Macs, the external drive must be attached via FireWire, while Intel-based Macs can boot from either FireWire or USB external hard drives.

Step 1

Connect the external drive to the computer you wish to boot. If the drive requires an external power source, make sure the power is connected and that the drive turned on. It is best to attach a USB drive directly to the computer, not through a USB hub.

Step 2

Eject any CD or DVD that may be in either drive. The key combination you will use later is also applicable for booting from a CD or DVD.

Step 3

Turn on or restart your computer. To restart a computer that is already on, press the power button once to enter the shut down-sleep-restart menu. You can also press "Control + Command + Power" to restart unconditionally. The "Command" key may be marked with a four-tipped curlicue figure, an apple or both. The "Control" key may be marked with an upward pointing caret.

Step 4

Press the key combination "Control + Shift + Option + Delete" and hold it down while the computer starts up. The "Option" key is usually marked with a diacritic that contains a diagonal line with a dash above it. The key might also be labeled "Alt." This key combination will force the computer to ignore the normal startup disk, which is usually an internal hard drive. The computer will instead boot from any attached bootable devices, such as an external drive or CD.

Tip

On certain models of Mac, you can simply hold down the "Option" key while the computer is starting up. This will launch the Startup Manager, where you can select the drive to boot from. This is available on iBook, MacBook Pro, MacBook, PowerBook, Mac Pro, Power Mac, Mac mini, iMac and MacBook Air.

If you are trying to boot a PowerPC from a large-capacity RAID FireWire drive, the boot might succeed only if the drive has a Logical Unit Number (LUN) of zero. On some FireWire drives, you can use software to configure the LUN. A simpler approach is to temporarily remove all other FireWire devices while you start up the computer from the external drive.

If you are using a Bluetooth keyboard on a PowerPC, your computer might not recognize the startup key combinations. If you experience this problem, use a non-Bluetooth keyboard until the computer boots up.

Warning

To boot from an external hard drive, the drive must use the partition type that is native for the Mac being booted. For Intel-based Macs, that is GUID. For PowerPC Macs, it is Apple Partition Map (APM). You can check the partition type using Disk Utility.

The hard drive must also contain an installation of a Mac OS that is no earlier than the OS that was shipped with your computer.

USB hard drives are only bootable with Intel-based Macs. To boot a PowerPC Mac from an external hard drive, you have to use FireWire.