Startup Disk Preferences on Your MacBook
Mac OS X displays the version numbers of each system and the physical drives where each system resides. Select the Network Startup icon on your MacBook if you want to boot from a System folder on your local network; typically, such a folder is created by your network administrator. If you’ve set up a Windows partition on your hard drive using Boot Camp, the folder appears with a Windows logo.
To select a startup disk, click the desired startup drive from the scrolling icon list.
If you’re planning on rebooting with an external Universal Serial Bus (USB), Thunderbolt or FireWire startup disk, that disk must be connected already, powered on, and recognized by the system before you display these settings.
Click Target Disk Mode to restart your MacBook as a FireWire or Thunderbolt external hard drive connected to another computer.
(This comes in especially handy when you’re upgrading to a new Mac and you need to move files between the two computers. You can also use it when the video card in one of your Macs decides to stop working. You can make an updated backup copy of the ailing Mac’s hard drive before sending it off for repair.)
You can also restart your MacBook and hold down the T key to invoke Target Disk Mode.
After you click a disk to select it, click the Restart button. Mac OS X confirms your choice, and your MacBook reboots.