A Macs Recovery Mode is for more than just reinstalling Mac OS X. Youll find many other useful troubleshooting utilities here you can use these even if your Mac cant boot normally. To access Recovery Mode, restart your Mac and press and hold the Command + R keys during the boot-up process. This is one of several hidden startup options on a Mac. Reinstall Mac OS X Most people know Recovery Mode as the place you go to reinstall OS X on your Mac. Recovery Mode will download the OS X installer files from teh Intenret if you dont have them locally, so they dont take up space on your disk and youll never have to hunt for an opearign system disc. Better yet, it will download up-to-date installation files so you dont have to spend hours installing operating system updates later.