Intel-based Macs support starting from an external USB storage device's volume that:
- Has been formatted with a GUID partition type
- Contains an installation of Mac OS X 10.4.5 or later, or Mac OS X 10.5 or later, which is compatible with (or shipped with) the Mac that the USB device is connected to. Note: You should not use a version of Mac OS X that is earlier ("older") than the version your Mac shipped with.
To start from a USB storage device that meets the above requirements, connect the device, then select it in Startup Disk preferences. Or, connect the device, restart, and immediately press and hold the Option key to access Startup Manager.
- It may be necessary to connect the USB storage device to an external power supply, depending on its power requirements.
- If you're using a MacBook or MacBook Pro, try using a different USB port if you cannot startup from the external USB storage device.
- On a MacBook Air, the computer may shutdown automatically as a preventive measure if a USB device with issues is connected, or if the USB port on the MacBook Air itself is damaged.
- If you use Boot Camp, you may not be able to boot natively into supported versions of Microsoft Windows XP or Windows Vista operating systems installed on external USB hard drive.
If you're using an external USB hub with several connected USB devices, the external USB storage device you wish to start from may not be recognized, or, may need to be connected directly into one of the computers USB ports instead. Some computer keyboards that include USB ports may not supply enough power to some external USB hard drives.
Tip: If your USB device isn't working, try following the troubleshooting steps here.