How Intel Macs with T2 chips, and Apple silicon Macs can be rescued from the apparently dead by connecting them to another Mac in DFU mode.
T2
Refurb, from a reseller, or buy private? What to beware of, what to check, and how to ensure the Mac you buy won’t lock you out.
How to use Share Disk in Recovery on an Apple silicon Mac, how fast it can be. Target Disk Mode on Intel Macs, and some less likely alternatives.
How to read and interpret firmware version numbers from the first Intel Macs to the latest Apple silicon models.
Details of new firmware versions for iMac 2019 models, those with T2 or Apple silicon chips. And how Apple assigns version numbers to iBoot.
One Thunderbolt port is the designated DFU port, to be used when connecting via USB-C in DFU mode, and to be avoided for external bootable disks. But how can you identify it?
You press the Power button, but nothing happens. Has your Mac suddenly died, does it have a problem you can fix easily, or is it just in DFU mode?
You’re sometimes advised to shut your Mac down, wait a while, and start it up again, instead of restarting it. Are they different, and if so when should you use them?
It throws away the encryption key for the Data volume, effectively wiping all its data. It doesn’t alter the System volume, SSV, or the macOS installed.
You’ve tried entering your password, but it doesn’t work. Should you try guessing? As that imposes increasing waiting times of up to 8 hours, maybe not. Try this instead.
The Eclectic Light Company 