Somona won't install even in DFU mode. The Mac “Mac” could not be restored. An unknown error occurred (21).

Question

Level 1

4 points

Somona won't install even in DFU mode. The Mac “Mac” could not be restored. An unknown error occurred (21).

I wiped my internal SSD as I was playing with Linux and ended up wiping my drive as I tried to reinstall the OS (Somona) as per How to revive or restore Mac firmware - Apple Support and the computer (2023 MacPro M2) has been completely useless after that.

I am using my Mac M1 Pro as the rescue mac also running lastest Somona (14.5), initially the process was failing with a cryptical "Corrupted Software" message over and over again until I figured out it was disk space issue ( I had only 20Gb free) but apparently Somona seems to need at least 30Gb free on the HD on the mac you use as a rescue but that's never mentioned in the procedure, why would It?

Once I overcame that, the process seems to go well until after extracting the Software and as it says it is preparing the Mac then I am greeted with "Unknown error 21" and everything I found about it suggests to change the source mac which for me is not an option (I just have 2) any idea on what the error is? Or how to get more details on it.

I have tried 2 original charging cables and I've bought a brand-new Belkin Thunderbolt4 cable and I've got the same results. The cable mentioned in the procedure is a broken link now but I found a newer version on Apple website that fits perfectly the specs but that is still on its way. Any suggestion is appreciated, I can't believe a modern mac can't simply boot from a USB and reinstall the OS, I tried that as well.

MacBook Pro 14″, macOS 14.5

Posted on Jul 4, 2024 11:35 AM

Reply

Answer 1

FelipeDeBene Author

Level 1

4 points

Jul 8, 2024 7:05 PM in response to FelipeDeBene

I resolved, it turns out apple procedure puts your MacBook into Recovery Mode, not DFU. Once I reached DFU it flowed like a charm.

Reply

Answer 2

Jul 5, 2024 4:43 PM in response to FelipeDeBene

A Thunderbolt Cable cannot be used for the DFU firmware Revive/Restore procedure....only a USB-C cable which only supports the USB protocol is supported as mentioned in the Apple DFU firmware Revive/Restore instructions you linked. So the Apple USB-C charging cable is the correct Apple cable to use....it is the Apple USB-C cable without any markings on the USB-C connector.

Think carefully about the error message you received....."Unknown error 21"....that should be a clue that Apple doesn't know either. In fact I have found most of Apple's current error messages are clueless and/or even completely misleading anyway. Unfortunately Apple doesn't seem to document their error messages (it is rare when Apple does document some of them).

The best you can do is making sure to disconnect all external devices from both Macs except for the USB data cable connecting them together. There is also still a chance you still may not have enough Free storage space on the host Mac.....I would suggest clearing up a bit more storage space since I have no clue how much storage space the DFU Revive/Restore process requires. Try using another USB-C port on the host Mac....especially one on the other side of the laptop.

Are you running any anti-virus apps, cleaning/optimizer apps, or third party security software on the host Mac? If so, then these apps may be causing the problem since these types of apps are known to cause all sorts of problem because they interfere with the normal operation of macOS. I guess you could try booting the host Mac into Safe Mode to see if you can still perform the DFU firmware Restore.....I have no idea if Safe Mode will prevent this from working.

Maybe you can use a Mac at an Apple Store as a host for the DFU firmware Restore process. Otherwise you will need to have Apple or an Apple Authorized Service Provider assist you with performing the DFU firmware Restore.

Reply

Answer 3

a brody

Level 10

85,366 points

Jul 4, 2024 11:38 AM in response to FelipeDeBene

Macs will have external media boot blocking if you added security on the firmware level using the T2 chip security login.

Macs can only boot off the same system that came with them or newer.

What is DFU mode?

The MacBook Pro is the notebook computer, where the Mac Pro is a desktop tower. Do you have a notebook or a desktop tower?

Reply

Answer 4

a brody

Level 10

85,366 points

Jul 5, 2024 3:30 PM in response to FelipeDeBene

Actually the T2 chip is on every Mac since its inception. A modified T2 chip is on the Apple Silicon, but it still is a chip which allows you to set a specific login on the open firmware boot screen. If this login is forgotten and its recovery keys are forgotten, then the Mac essentially becomes a brick. It may not be called T2 on Apple Silicon, but it is still the same concept of "Secure Enclave":

https://recoverit.wondershare.com/mac-tips/t2-chip-vs-m1.html

Reply

Answer 5

FelipeDeBene Author

Level 1

4 points

Jul 4, 2024 1:25 PM in response to a brody

It’s a m2 MacBook Pro so there’s no t2 chip. As per Mac computers with the Apple T2 Security Chip - Apple Support

Reply

Answer 6

a brody

Level 10

85,366 points

Jul 6, 2024 1:33 PM in response to FelipeDeBene

M2s still have a similar Secure Enclave.

Reply