There are two ways to restart a Mac that’s already running: you can either use the Apple menu command Restart (or an equivalent in Terminal), or you can shut it down, leave it a few seconds, and start it up afresh in what’s known as a cold boot. This article examines whether there’s any difference, and which you should prefer.

Differences

When a desktop Mac starts up from cold, its power supply starts providing power to its internal components, and when the boot process starts they are all initialised from their power-off state. There’s one small exception to that in the internal clock, possibly part of the Always On Processor (AOP) in Apple silicon Macs.

It’s commonly held that a regular (or warm) restart doesn’t interrupt the power supplied to internal components, as Intel Macs may not undergo power-on self-testing (POST) when only restarted. That doesn’t appear to be correct, though, as it’s easily demonstrated that Intel Macs with T2 chips do undertake a POST during a restart. Apple silicon Macs don’t appear to perform a similar form of POST. Neither does there appear to be any difference for peripherals, which are shut down before being started up again whether the Mac is booted warm or cold.

It’s also most unusual to attempt a true cold boot, particularly in a laptop. To remove all power from a desktop Mac requires it to be disconnected from its mains power supply, and for a laptop the internal battery also needs to be isolated or removed, something only performed during service by a technician.

What is clear is that the period during which power is turned off is very different: during a warm restart that’s only a second or two.

Received wisdom is that this allows data to be retained in memory through a warm restart, but that it’s wiped clean after a cold boot. This was first documented by Alex Halderman and others, who used it to perform ‘cold boot’ attacks to recover encryption keys. They demonstrated that RAM contents were still recoverable many seconds after shutting down, and could be used to recover encryption keys and other critically secure data. A great deal has changed since that research was undertaken. As far as the protection of secrets is concerned, all modern Macs have secure enclaves and use Extended Anti-Replay Technology (xART) to prevent that.

It’s thus plausible that there could be some differences in hardware between a restart and a cold boot, but careful comparison of log entries during booting from a Mac’s internal SSD fails to reveal any significant differences there.

How long to wait?

There appears to be no useful evidence for recent and current Macs to determine how long you should wait between shutdown and a cold boot. Periods given range widely, from a few seconds to a minute or more.

Experience

There are many reports of cold boots resolving problems that weren’t affected by a warm restart, but those are generally if not exclusively for Intel Macs, and most usually those without T2 chips.

I have only once experienced a problem in an Apple silicon Mac that required a cold boot, and that occurred a couple of years ago with a dual-boot setup that wouldn’t restart into its primary macOS on its internal SSD, but would do so when started up with a cold boot.

Cold booting is sometimes recommended following more severe problems, such as a freeze or kernel panic. As the normal way to recover from a frozen Mac is to force shutdown using the Power button, there is no option other than a cold boot. Kernel panics should normally result in an automatic restart, the behaviour that demonstrates that a panic took place, and only after that restart is the panic confirmed in an alert, from where the panic log can be accessed. There is no option to perform a cold boot until after the restart. Apple silicon Macs are almost invariably cold booted into Recovery or Safe mode, as those require a long hold of the Power button.

Thus the choice between a restart and a cold boot is usually determined by the problem that has occurred.

Summary

• Perform a cold boot by shutting down, waiting at least ten seconds, then starting up again.
• A cold boot may sometimes be more effective at fixing problems than a restart.
• If you have tried restarting your Mac to address a problem and that didn’t help, try a cold boot.
• Cold booting may be more effective at addressing problems with external boot disks, or possibly with other peripherals.
• We don’t fully understand why a cold boot might differ in effect from a restart, but it could.
• There is no evidence that a Mac should always, or usually, be cold-booted rather than restarted.