macOS Recovery installs different versions of macOS depending on the key combination you press while starting up. Hold down one of these combinations immediately after pressing the power button to turn on your Mac. Release when you see the Apple logo or a spinning globe.
This seems to be new in macOS 10.12.4, and it came in handy for me today. As with the last several updates, the 10.12.4 update completed on my main Mac but then the Mac got stuck near the end of the boot process. (I had recently done a clean install in the hope that it would clear out whatever was causing the update problems, but alas it did not.) Unlike prior updates, rebooting and safe booting didn’t help. The Mac just wouldn’t boot into my main account or even a guest account. It looked like I’d need a fresh macOS install.
The new Option-Command-R keyboard shortcut lets you do an Internet recovery of latest macOS that is compatible with your Mac. So, rather than boot from a slow hard drive, wrangle the Mac App Store, download the installer, and then launch it, I just held down the keys and let it download and install all at once. The time estimate was way off (under), but other than that it worked smoothly and got my Mac booting again.
I’ve now got a folder at /.PKInstallSandboxManager-SystemSoftware, presumably left behind by the incremental update.
sudo rm doesn’t seem to be able to delete it. It looks like I would need to turn off System Integrity Protection to do that.
Update (2017-03-30): Thomas Brand:
Internet Recovery is now a convenient time-saver, skipping the unpatched versions of system software and restoring the latest Mac OS. I am a fan just as long restoring older versions of Mac OS are still an option. One of my least favorite aspects of iOS is that you cannot install the version of system software that originally shipped with your device.
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