Firmware updates seem to be coming fairly regularly now, and over the coming few months, with Mojave on its way, we can expect at least one more. What can you do when they go wrong, though?

A failed firmware update can be seriously bad news, even for the seasoned Mac expert. Macs can all too easily get locked into a boot-loop cycle, in which they try to start up, crash horribly before they’ve got anywhere, then repeat the process. This affected some with earlier Touch Bar models of MacBook Pros when their T1 firmware update didn’t work properly, for instance.

The good news, as far as EFI firmware goes, is that all recent models, after about 2010, should be self-repairing. Some should sound a —…— (3 long, 3 short, 3 long) tone then automatically recover their firmware, and those which are mute – such as MacBook Pro models from 2016 onwards – should just get on with their recovery in silence.

Models from earlier than 2010, notably the MacPro5,1, need to be started up from a Firmware Restoration CD. That is detailed in this earlier article.

Macs which have a T2 chip – the iMac Pro and MacBook Pro 2018 with Touch Bar – are, as you probably guessed, different. The T2 is much more than just a glorified SMC, and can stop that Mac dead in its tracks.

To restore the T2 firmware on an iMac Pro or MacBook Pro 2018 with Touch Bar you will need another Mac running High Sierra 10.13.5 or later, Apple Configurator 2 app (free from the App Store), and a suitable cable to connect the Macs back to back. Apple prescribes:

• a “supported” USB-A to USB-C cable,
• a “supported” USB-C to USB-C cable supporting both power and data, which can be used with a Thunderbolt to USB-C adaptor, or
• a “supported” Thunderbolt cable.

The Mac being used to recover the T2-equipped Mac must be running off mains/AC power and connected to the internet. Then, ensure that you have quit iTunes (if running), open Apple Configurator 2.6 or later, and connect the two Macs with your cable.

On an iMac Pro, disconnect the iMac Pro from power and connect the cable to the Thunderbolt port nearest the ethernet port. Hold the iMac Pro’s Power button down, connect the iMac Pro to mains power, and keep holding the Power button for another 3 seconds.

On a MacBook Pro, press the Wake/Sleep button for 5 seconds to shut it down, and connect the cable to the Thunderbolt port further from the display hinge on the left side of the MacBook Pro. Hold the Power button down, and press these three keys together for 3 seconds: the Right ⇧ Shift key, Left ⎇ Option key, and Left ⌃ Control key.

On the Mac running Apple Configurator, select the T2 Mac in the device browser in that app. Choose Actions > Restore, then click Restore. Wait for the restore to complete, which is accompanied by the Apple logo appearing, then disappearing, following which the T2 Mac should reboot. If it doesn’t, or the restore fails, you should try again.

Once the T2 Mac has successfully rebooted, quit Configurator, unplug the cable, and breathe a huge sigh of relief.

The alternative (particularly if you don’t happen to have a suitably-equipped second Mac) is to take your shiny new iMac Pro or MacBook Pro off to a Genius Bar to perform this complicated process. Thankfully this should be very rare needed.

Apple details this here for the iMac Pro, and here for the MacBook Pro.

Thanks to @tperfitt for drawing attention to Apple’s docs.