Howard Oakley’s latest article on this topic:
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As usual, a lot of what Howard had to say tested my ability comprehend (but please keep posting links).
In case anyone else got frustrated wondering what DFU mode meant, here is what I found after searching many pages. This was in a response from Howard in the comments of a different article.
The abbreviation stands for Device Firmware Update, but it doesn’t any more mean that with M1 Macs. They’re not devices like iOS devices, it deals with the whole of macOS and not just their iBoot firmware, and – as it can also downgrade an M1 – it isn’t confined to updating.
So it doesn’t actually mean what the abbreviation spells out. Perhaps it’s now better just to use the letters?
DFU is a term used by many embedded devices and chipsets.
It refers to a mechanism (any mechanism) that can be used to install/upgrade software on a device when normal mechanisms (e.g. Software Update) can’t be used. Possibly because the device’s system software can’t boot normally.
For an iOS device, you enter DFU mode using some magic sequence of button-presses. At which point, you connect it to a computer via a USB cable and update/restore it (via iTunes on old Macs and PCs or the Finder on newer ones).
For an Apple Silicon Mac, there’s something similar, but it’s not the same.
An Apple Silicon Mac has its lowest-layer boot-loader (the One True Recovery, or 1TR mode). This is used, among other things, to let you reinstall macOS when you have no functioning macOS system, or for booting from external drives.
If the 1TR APFS container gets deleted or otherwise damaged, however, there is no longer any way to boot such a Mac. The ROM-based Internet Recovery mode that Intel Macs have doesn’t exist on Apple Silicon. The solution is the Mac version of DFU mode.
To use Mac DFU mode, you need another Mac that is running the latest version of Apple’s Configurator tool. You then connect the two Macs with a USB-C cable and power-on the dead Mac with a magic sequence of button-presses. Then you use Configurator to install/update firmware, restore/install the 1TR container, RecoveryOS and macOS.
Then you can use Migration Assistant to restore your data from a backup (hopefully, you had one).
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