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.
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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.
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.
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.
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.