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Let's take a look at the Startup Disk system preference. System Preference. Startup Disk. If you have only one hard drive attached to your Mac, and that's often the internal hard drive, it will start up from that disk every time you start it up. However, you have the option to start it up from a different disk if you've attached a different disk to your Mac. Alternatively, if you've partitioned your disk into two or more volumes, you can install the OS on another partition.
So for example, you might have Lion on one partition and you might have Snow Leopard on another one, if your Mac supports booting from Snow Leopard. In this case, we have two hard drives attached to this Mac. There is the internal drive here, which we see as Macintosh HD, and I have an external FireWire drive, which is this Porsche drive. You notice it too has a version of OS X on it. If you don't have a version of OS X on a drive, it will not appear in the Startup Disk system preference because you can't boot from it.
If you want to boot from a different volume, it's easily done. All you have to do is select that volume and then click Restart. The next time your Mac boots, it will boot from the volume that you've chosen. One other option here is Target Disk Mode. Here is how it works. You have two Macintoshes. You string a FireWire cable between the two, Mac A and Mac B. Let's say I want to boot Mac B into Target Disk Mode. What I do is I restart Mac B while holding down the T key on the keyboard.
When I do that, the Mac will boot and it will show a FireWire symbol. This indicates that it's in Target Disk Mode. On Mac A, I'll see Mac B's hard drive as an external hard drive. Okay, so what good is this? Well, really it's good for troubleshooting. If I'm having a problem starting up from Mac B because there it seems to be something wrong with its hard drive, I can boot Mac B into Target Disk Mode. Then on Mac A, I can troubleshoot its hard drive using diagnostic and recovery utilities.
This is a good troubleshooting tip to keep in your back pocket, should you have two Macs. And honestly, that's it. That's Startup Disk. Now this isn't the only way that you can start up from another drive if you have more than one bootable drive or partition. Here's the secret. Restart your Mac and hold down the Option key. When you do that, you'll see every volume that can boot on your Mac screen and then to boot from that volume, simply select that volume and then login, and that's Startup Disk.
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