Mac recovery - the M1 change
Backup your data prior to using any recovery routine.
Prior to Lion's July 20, 2011 release there was no built-in support for recovery. This tip discusses alternatives to recovery:
https://discussions.apple.com/docs/DOC-250003506
Upon until November 11, 2020, recover options remained consistent:
command-R would restore from the operating system that was currently loaded on a system that was healthy with the latest current release of that operating system. Even if 10.14.3 was installed, you would be able to get 10.14.6 loaded fresh on the computer. By "fresh" that means that the components would be updated, and any missing components that were deleted from the standard install would be added back in. If you had corruption in your preferences or caches, they would still remain, and that's where often only internet restore would resolve the issue. If issues with slowness or crashing persist, before you attempt a wipe, follow the tip here to determine if anything in the background is not working right:
https://discussions.apple.com/docs/DOC-250002463
command-shift-option-R offered internet restore, which loaded the system that shipped with the Mac, unless it was recently repaired by Apple and swapped in with a new logicboard. If this is different than the OS currently installed, you will want to use Disk Utility to erase and prepare the Mac for the older system. Any Apple software that was upgraded to newer versions than the operating system will have updated libraries such as Photos and Music, so don't expect those libraries to work again until you can restore the the OS that they were last updated to.
Often though if the serial number indicated that an older operating system was possible, you could still restore it from a bootable external copy of that older system, using the upgrades Apple has for download on
https://support.apple.com/en-us/HT211683 as long as the system was old enough for the upgrade to be the base installation. Keep in mind, Macs older than September 25, 2017's release of High Sierra supported HFS Extended Journal for installing Sierra or older operating systems and those systems can't see High Sierra or newer systems on SSDs, or APFS formatted systems with High Sierra or newer, except over a network.
With the new M1 chip, released on certain new Macs starting November 12, 2020, a new era in internet recovery and recovery began.
Hold the power down until "Loading startup options" appears. Then select Options, and Continue.
You will need your administrator password to resume. Internet recovery works if your Mac is connected to the internet when you do this routine.
For more details about this routine, visit:
https://support.apple.com/guide/mac-help/macos-recovery-a-mac-apple-silicon-mchl82829c17/mac
