Since OS X Lion, installing the Mac operating system includes creating a Recovery HD volume, which is hidden on your Mac's startup disk. In an emergency, you can boot to the Recovery HD and use Disk Utility to troubleshoot the hard drive, go online and search for information about the problems you're experiencing, or reinstall your Mac operating system.

For more information about using the Recovery HD volume, see our guide on using the Recovery HD volume to reinstall or troubleshoot macOS.

Apple has also created a utility called OS X Recovery Disk Assistant that can create a copy of the Recovery HD on any bootable external drive you have connected to your Mac. This is good news for the many Mac users who want to have the Recovery HD volume on a separate drive than the startup volume. However, the utility can only create the Recovery HD volume on an external drive. This rules out all the Mac Pro, iMac, and even Mac mini users who may have multiple internal hard drives.

With the help of some hidden macOS features, you can create a recovery HD volume anywhere you want, including on an internal drive.