OS X, as you’ve probably heard, no longer comes on a DVD. When your Mac won’t even start up, there’s no DVD to insert as a backup startup disk, as there used to be. That’s a scary place to be; if your hard drive is sick, then everything on it may be inaccessible to you. What you need is an emergency drive that can repair the hard drive and get you back in operation.
Apple is way ahead of you on this one. It offers three simulated repair disks: three ways to supply disk-repair and system software-repair tools in times of crisis.
Here’s the sneaky surprise: When you install Yosemite, it automatically creates an invisible hidden “hard drive” (a partition of your main drive that has its own icon) called Recovery HD. It’s a 650-megabyte “drive” that’s generally invisible to you. It’s even invisible to Disk Utility, so even if you erase your hard drive, the Recovery HD is still there to help you.
But if you can’t seem to start up your Mac properly, or if your hard drive is acting a little flaky, you’ll be glad you had this “separate” startup disk. On it, Apple has provided some emergency tools for fixing drive or software glitches, restoring files, browsing the Web, and even reinstalling OS X.
The OS X installer creates the Recovery HD partition only if your startup drive is an internal drive. In certain other situations, it won’t create the partition—for example, if you’re installing OS X onto a RAID volume or one with a non-standard ...