These OSX startup key combinations are valid on a MacBook Pro (Early 2011). I had the misfortune of OSX 10.10 upgrade problems and had to use various commands to recover and perform a clean install of OSX 10.10.
- Startup Manager (Press “Option” key during startup) will let you choose to boot from a specific OSX volume, USB volume, or network volume.
- Internet Recovery (Command + Option + R) will reinstall OS version that originally came with computer.
- Recovery Partition (Command + R) will reinstall OS version that is currently installed on your hard disk.
- Safe Mode (Press “Shift” during startup)
- Verbose Mode (Command + V) will display boot messages.
- Save + Verbose Mode (Shift + Command + V)
- Single User Mode (Command + S) will boot to a single user shell
- Diagnostics (Option + D) will download and run Apple Hardware Test
- Reset NVRAM (Option + Command + P + R)
- Reset SMC (Shift + Control + Option + Power)
More key combinations are listed in Apple article 201255, such as:
- Boot CD (Press “C” key during startup) will start from bootable CD/DVD/USB.
- Eject CD (Press “Eject” or “F12” or hold mouse button or hold trackpad button) to force MacBook to eject disc.
- Net Boot (Press “N” key during startup) will attempt to start from a network server.
- Net Boot (Option + N) will start from a network server
- Target Disk Mode (Press “T” key during startup) will allow the hard disk in this computer to be used as an external hard disk in another computer, when the two computers are connected via FireWire.
The help article above also includes the following tip: “For the best experience with startup keys, press the keys immediately after the startup tone plays.”