If your Mac has multiple bootable disks—via either a partitioned internal disk or external FireWire drives—you’re probably very familiar with the Startup Disk System Preferences panel. As you’d guess from its name, this panel controls which of your bootable volumes is used when you boot your Mac. You’re probably also familiar with the Startup Manager, which appears if you hold down the Option key during startup. The Startup Manager allows you to override the default boot disk and choose another to boot from for the current session—but doing this won’t change the default boot disk.
But what if you want to permanently start booting from the selected volume? Typically, you’d let the Mac boot from your chosen disk, then go into the Startup Disk System Preferences panel and set that same disk as the startup disk. Here’s a shortcut that may (or may not) work for you, depending on which particular Mac you own. Boot your machine with the Option key down to invoke the Startup Manager, then hold down the Control key prior to clicking on a volume to boot. (You may have to use the left and right arrow keys to select a volume first.)
If this trick is supported on your Mac, you’ll notice that upward-pointing arrow—the one that points to the selected disk—will change into a circular arrow instead, indicating that the chosen disk will not only be used to start your Mac this time, but that it will also become the default boot disk. I tested this on my 2006 MacBook Pro, and it did not work. It did, however, work on our 2008 20-inch iMac. It’s also known to work on the MacBook Air, and probably any other recent Mac. If you’ve got a multi-disk-bootable Mac, give it a try and see if it works for you—if it does, it can be a nice little timesaver for those times you’d like to permanently change the startup disk.