Macs have different modes in which they can start up. This short guide explains which does what, how to start up in that mode, and a link to a fuller account.

Before trying to start up or restart in any except for normal user mode, it’s best to connect your keyboard (and mouse/trackpad) using a USB connection to your Mac, such as the charging cable for wireless models. Although many Macs will start up in special modes using Bluetooth alone, this has become increasingly unreliable. There’s nothing more frustrating than trying to start up in Recovery Mode, say, and finding your Mac ignores you completely.

You want to check that your hardware is OK: Diagnostic Mode

On all models released since June 2013, this goes straight into a dedicated tool which performs basic hardware diagnostics. Have a pen(cil) and paper handy, as it returns a code which you’ll need to record. To perform this reliably, disconnect all peripherals except the keyboard, mouse/trackpad, any external display, and any Ethernet network cable. You’ll also need to disable any firmware password first, and put a MacBook Pro, MacBook Air or MacBook onto mains power before starting. This is best performed after shutting down, rather than restarting.

Hold the D key during startup to engage Diagnostic Mode.

Full details are here.

You want to check and repair your startup disk, reinstall macOS, restore from a backup, change firmware password or Secure Boot settings: Recovery Mode

This starts the Mac up from its normally unmounted Recovery Volume, providing a suite of tools to tackle these and other tasks. The tools include Disk Utility, macOS Installer, Restore from Time Machine Backup, Terminal, Network Utility, Firmware Password Utility, and Startup Security Utility. Although you can’t add your own tools to this list, when in Recovery Mode you can mount your normal startup volume and access tools on it using Terminal.

Hold the Command and R keys (with/without Option and Shift according to the variant required) during startup to enter Recovery Mode.

A guide to the different variations of Recovery Mode is here.

You want to flush user caches and disable third-party extensions, etc.: Safe Mode

This does three things: in Mojave and earlier. It checks your disks and all snapshots, which can take 30 minutes or more, and may automatically make necessary repairs, but doesn’t report the results, and skips this entirely in Catalina. It disables a long list of Apple’s kernel extensions, and doesn’t load any third-party extensions or daemons. It also flushes and rebuilds user caches and OpenDirectory databases. This can clear some strange problems, and is a valuable tool for diagnosing problems with third-party extensions, etc.

However, the checks on snapshots in High Sierra and Mojave can take a very long time, and it isn’t a useful way of checking your startup volume, which is much better performed using Disk Utility in Recovery Mode.

Hold the Shift key during startup to enter Safe Mode.

This article provides full details of Safe Mode in Catalina, and additional information is here.

You want to select which startup volume to boot from: Startup Manager

Hold the Option key during startup to display a list of different volumes which you can choose to start up from. When you have selected one, startup will proceed from that volume.

You want to see details of the startup process: Verbose Mode

Hold the Command and V keys during startup to see scrolling log information about that startup as it proceeds. On most models, this now scrolls past so quickly that it’s better to browse the log after startup is complete using a utility like Consolation. However, it can occasionally give immediate insight into startup problems.

You want to reset the SMC and/or NVRAM

Resetting the System Management Controller (SMC, a function also performed by the T2 chip in recent models) can fix various problems, such as difficulty sleeping or waking from sleep, cooling fans stuck on, and more. Full details for doing this are given here.

Resetting the Non-Volatile RAM (NVRAM, formerly known as PRAM) can fix some other problems, and is normally performed after resetting the SMC. Hold the Command, Option, P and R keys until the startup chime sounds a second time, until the display light comes on a second time, or until the Apple logo appears and disappears for the second time. If you hold the keys too long, the Mac cycles through a second reset; if you don’t hold them long enough, the reset won’t be performed. This is best performed after shutting your Mac down, rather than after a restart, and requires any firmware password to be turned off first. Further details are given here.

You want to recover your Mac from a failed firmware update

Firmware updates are now only performed as part of a macOS upgrade, update, or a security update. If such an update goes wrong, your Mac can get stuck in an endless reboot loop, in which it tries to start up, suffers a kernel panic, then tries to start up again. This is now most likely to occur in a Mac with a T2 chip: details of restoring T2 firmware are given here, and additional information and cautions are here.

Mac models prior to October 2016 sound nine tones – three long, three short, and three long – when started up with corrupt EFI firmware, and should then enter EFI firmware recovery automatically, displaying a progress bar. Once complete, the Mac should restart automatically with its firmware restored. More recent models without a T2 chip should perform a similar sequence, but without sounding any tones to indicate that they’re starting the process of recovery. If recovery appears incomplete, try resetting the SMC and NVRAM. Further details are here.

You want to set your Mac into Target Disk Mode

Hold the T key during startup to force that Mac to provide its startup disk as an external disk to another suitably connected Mac.

You want to set your iMac into Target Display Mode

The requirements for this to work are very restrictive. Full details are given here.

You want to run fsck at the command line, or use another command tool on a minimal running version of macOS: Single-User Mode, now replaced by Terminal in Recovery Mode

In the traditional SUM, macOS boots up so far but stops short of loading the GUI. This makes it ideal for running some command tools such as fsck for checking and repairing disks. However, this is more complex with Fusion Drives, and isn’t allowed on Macs with a T2 chip. This is explained fully in this article.

Hold the Command and S keys during startup to enter Single-User Mode, or Command and R keys for Recovery Mode, or Command, R and S for ‘SUM in Recovery’. Note this is only intended for use on Macs with a T2 chip: if you use it on a Mac without a T2 chip, it will probably enter normal SUM. If you have a Retina display, the text will be so tiny that you’ll need a magnifying glass to read it.

A short guide to startup, shutdown and other key commands is here.