Put the computer into DFU mode (as described in another tip here) and on the good computer you can either revive or restore the "broken" machine.
Reviving is faster and doesn't erase the broken Mac. It might not be able to repair the broken computer, but try it first.
Reviving
In the Finder, from the DFU window, click "Revive Mac", then click "Continue" to confirm this. A progress bar should show the process running. When it is done, the broken computer should restart, or press the power button to start it.
If you are asked to select a volume to recover, choose the built-in disk (usually called "Macintosh HD") and click "Next", if asked, select a user and enter the password, then click "Next", then "Restart". If the revive was successful the Mac should finish starting up.
Restoring
If the revive fails, try these steps to erase it and restore it to factory settings. Note that this erases everything on the broken computer, so only do this if everything else has failed.
In the Finder's DFU window click "Restore Mac", then click "Restore and Update" to confirm. A progress bar will show as the restore is progressing. When it is complete the broken Mac should restart automatically, or press the power button to start it.
Choose a wifi or wired connection to connect to the network, older machines will install an OS from the internet, Apple Silicon Macs should have an OS already installed. Finally, complete the setup normally.
I usually write a blog post about once a week. The latest post can be viewed here: What Is Anything?: Use language in an honest way to communicate instead of obfuscating. (posted 2025-08-07 at 20:27:52). I do podcasts too!. You can listen to my latest podcast, here: OJB's Podcast 2025-07-18 Too Many Road Cones: I think it is clear, there are too many road cones!.
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