© 2021 by Lawrence I. Charters, Vice President, Strait Macintosh User Group
The Macintosh operating system has a number of startup key sequences that are very useful in tracking down problems.
All of these sequences should be triggered at startup. In other words, press the keys before you start the machine. “Start” can mean turning on the power, or restarting the machine after it is on. Make sure your Mac is plugged into power; many of these actions can take a long time.
⌘ – Symbol for the Command key
⌥ – Symbol for the Option key
⇧ – Symbol for the Shift key
Command-R. For several years, Apple has installed an invisible “recovery” system on the disk when you install macOS. The recovery system (sometimes called the “recovery disk” or “recovery partition”) allows you to do a limited number of things: 1) You can run Disk Utility to check on the health of your disk drive; 2) You can reinstall the operating system (just the version you have; you can’t do an upgrade this way); or 3) you can restore your disk from a Time Machine backup.
⇧⌥-R — Shift-Option-R — should only be used if you tried ⌘-R without success. ⇧⌥-R triggers the use of the recovery system across the Internet. You have the same options you have with ⌘-R, but instead of loading the recovery disk from your computer, you are running a recovery disk from an Apple server across the Internet. Obviously, this requires an Internet connection, but even with a decent Internet connection, it is quite slow. Be patient. Read a book. Make dinner. Watch a movie.
⇧ – Shift – Start up in safe mode. Safe mode is a quick way to see if a problem is with your user account, or some piece of software that loads when you start up in your user account. Only use this for diagnostics; don’t do any work while in safe mode, as your Mac literally is not operating as usual.
⌥ – Option – startup to Startup Manager. Startup Manager will allow you to select from available startup disks, if you have more than one. Hint: the recovery system is displayed in the Startup Manager as an available disk.
D – Holding down the D key while starting up triggers Apple Diagnostics utility. This utility performs a series of hardware tests on your Mac. You should disconnect all external peripherals except your mouse, keyboard, display, and Ethernet cable (if you are using Ethernet). The tests seem a bit cryptic; they were originally designed for use by Apple support engineers.
Option-D is essentially the same as pressing the D key at startup, but it triggers Apple Diagnostics across the Internet. You obviously need an Internet connection, and it takes longer, even on a fast Internet connection.
Pressing T at startup brings up your Mac in Target mode. This is only useful if you have another Mac. It allows you to bring up the Mac as if it was a disk drive, rather than a computer. If your two Macs are connected via FireWire or Thunderbolt or USB-C or Ethernet cable (depending on model), you can use one Mac to run diagnostic tests on the hard drive of the other Mac. Obviously, the most useful tests are those you can run with Disk Utility.