Starting a computer from an external hard drive is useful when you need to make system configuration changes, partition the internal drive, troubleshoot critical problems, format the system's main storage drive, or reinstall the operating system. You can boot the system via an external memory drive in both Windows and Mac.
Steps
Method 1 of 3: Windows 8
Step 1. If you are using a touch screen device, slide your finger across the screen, from the right side towards the center, then choose the "Settings" option
If you are using a standard device, move the mouse cursor to the lower right corner of the screen, then select the "Settings" item from the panel that appeared
Step 2. Press the "Shutdown" button, then choose the "Restart" option from the context menu that appeared
Step 3. While the computer restarts, hold down the "Shift" key on your keyboard
Step 4. Choose the "Troubleshoot" option from the Windows 8 menu that appeared on the screen
Step 5. Select the "Advanced Options" item from the next menu screen
Step 6. Choose the "UEFI Firmware Settings" option
Step 7. At this point choose "Restart"
You will then have access to the computer's BIOS menu.
Step 8. Use the directional arrows on your keyboard to select the "Boot" menu
Step 9. Change the "Mode" field value from "UEFI" to "Legacy"
Step 10. Now select the option to restart the computer, then quickly and repeatedly press the "F2" function key to access the BIOS again, but this time in "Legacy" mode instead of "UEFI"
The key to press to access the BIOS menu may vary depending on the manufacturer and model of the computer. For example, in some cases it is necessary to press the function key "F12" or "F5" instead of "F2"
Step 11. Use the directional arrows on the keyboard to select the "Boot" menu, then change the order of the memory drives used to load the operating system, so that the first (ie the default) is represented by the external hard drive that you connected to the computer
Step 12. Connect the external hard drive you wish to use to one of the free USB ports on your computer
Step 13. Restart your computer
At this point the BIOS will load the operating system inside the USB hard drive connected to the computer.
Method 2 of 3: Windows 7, Windows Vista, and Windows XP
Step 1. Turn on your computer
Step 2. Connect the external hard drive you wish to use to one of the free USB ports on your computer
Step 3. Go to the "Start" menu, then click the down arrow button next to "Shut Down"
Step 4. Choose the "Reboot System" option
Your computer will restart automatically.
Step 5. Press the appropriate keyboard key to enter the system BIOS
The key to press varies depending on the manufacturer of the computer. For example, you will need to press the "F12", "F2", "F5" or "Esc" function key.
Step 6. Use the arrow keys on the keyboard to select the "Advanced Settings" item
Step 7. Locate and choose the option named "Boot order" or "Boot sequence"
Step 8. Select the menu item for the external storage drive to make it the first boot device on the system
Step 9. Now save your changes and exit the BIOS
Step 10. Restart your computer
The BIOS will load the operating system on the USB hard drive connected to the computer.
Method 3 of 3: Mac OS X
Step 1. Connect the memory drive or external hard drive to your Mac
Step 2. Enter the "Apple" menu and choose the option to restart the computer
Step 3. Press and hold the "Option" key on your keyboard as soon as you hear the beep that indicates the system is restarting
The OS source selection menu will be displayed.
Step 4. Choose the name of the external hard drive you have connected to your computer
At this point the Mac will load the operating system present in the external memory drive.
