When trying to install Yosemite to a fresh hard drive a couple of weeks ago, I realized my knowledge of OS X startup key combinations was very lacking. I did a little research, and found the chart displayed below from Apple’s Support pages (To see the original document, click here). All the OS X startup key combinations and their uses can be viewed.
Note: The OS X startup key combinations are entered AFTER starting the computer. Keep holding key(s) until the mode or process you want starts.
| Shift ⇧ | Start up in Safe Mode. |
| Option ⌥ | Start up to Startup Manager. |
| C | Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media). |
| D | Start up to either Apple Hardware Test or Apple Diagnostics, depending on which Mac you’re using. |
| Option-D | Start up to either Apple Hardware Test or Apple Diagnostics over the Internet. |
| N | Start up from a compatible NetBoot server. |
| Option-N | Start up from a NetBoot server using the default boot image. |
| Command (⌘)-R | Start up from OS X Recovery. |
| Command-Option-R | Start up from OS X Recovery over the Internet. |
| Command-Option-P-R | Reset NVRAM. Release the keys after you hear the startup sound again. |
| Command-S | Start up in single-user mode. |
| T | Start up in target disk mode. |
| X | Start up from an OS X startup volume when the Mac would otherwise start up from a non-OS X startup volume. |
| Command-V | Start up in verbose mode. |
| Eject (⏏), F12, mouse button, or trackpad button | Eject removable media, such as an optical disc. |
