Is there still something amiss in the M1 firmware?
Wonderful though the M1 Macs new Recovery system is, theres a problem which has been plaguing a few users since the first of the new models shipped late last year. When in Recovery (or elsewhere) you erase your Macs internal storage to reinstall macOS, just when you start that reinstallation, up pops up the error message: An error occurred while preparing the update. Failed to personalize the software update. Please try again. I explained this, and linked to Apples support page, in this article on 26 November, almost seven months ago, yet users still encounter this in macOS 11.4 and in current beta-releases. Perhaps inevitably, Apple neither explains what is going wrong, nor does it tell us how we can avoid this from happening, other than by avoiding erasing internal storage in this way. Neither does it make clear that the erasure doesnt actually erase the whole disk, only its macOS container. This still leaves two other containers, with the firmware and recoveryOS on them, which can only be completely erased when you boot your M1 in DFU mode and restore from an IPSW image, something Apple details in this article from the Configurator Help book. The evidence from Apples advised solutions only strengthens the case that this is a continuing bug in the firmware or recoveryOS, or possibly in the Big Sur installer. Its a serious bug too: although it occurs infrequently, solutions are quite technically challenging and the only way to recover that Mac. Apples primary solution is to revive or restore the whole internal SSD with the Mac in DFU mode, which requires a second Mac running at least Catalina 10.15.6 and a suitable cable. In practice, of course, reviving the firmware is insufficient, as you also need to install macOS to go any further. At least a DFU mode restore is quite quick and painless, however daunting it might seem at first. If you cant do that, solutions proposed rely on a hidden Erase Mac command initiated through Terminal in Recovery, followed by reinstalling Big Sur. As these methods arent described elsewhere, Ill explain them here. Erase Mac in Recovery  Normally, erasing your Mac in Recovery only wipes the macOS container, and doesnt clear the other two containers. It appears that theres one manoeuvre you can use in Recovery to go further than that. Open Terminal there, and type in the command  resetpassword  following which you should click in the Reset Password window to bring it to the front. Then open the Recovery Assistant menu, and select the Erase Mac command. That opens another window titled Erase Mac, in which you should confirm by clicking Erase Mac again. Your Mac will restart automatically, and will then prompt you to select a language. It will inform you that macOS needs to be reinstalled, which you should continue with. Your Mac then activates by connecting to Apple over the Internet. Extraordinarly, Apple recommends that in this case, if you have seen the warning about failing to personalise, you should repeat these steps a second time, starting with the resetpassword command. Even then, youre still not done, as you have yet to reinstall macOS. To do that, Apple recommends one of three actions:  Reinstall Big Sur using the main Recovery window. Restart from a Big Sur bootable installer, if you happen to have one available. Reinstall using Terminal in Recovery, as explained below. Reinstall macOS in Terminal  The official procedure for reinstalling using Terminal in Recovery isnt simple either. Youll need to enter the following commands  cd '/Volumes/Untitled'  mkdir -p private/tmp  cp -R '/Install macOS Big Sur.app' private/tmp  cd 'private/tmp/Install macOS Big Sur.app'  mkdir Contents/SharedSupport  curl -L -o Contents/SharedSupport/SharedSupport.dmg http://swcdn.apple.com/content/downloads/55/59/071-00696-A_4T69TQR1VO/9psvjmwyjlucyg708cqjeaiylrvb0xph94/InstallAssistant.pkg  These create a temporary folder within a macOS installer to contain Install Assistant, and to download that. Once that has completed, type into Terminal the command  ./Contents/MacOS/InstallAssistant_springboard  to start the process of reinstallation. Choose macOS version  The other important issue which Apple doesnt mention is which version of macOS you should install after all this. It appears most likely that this problem occurs when you have had a beta-test version of macOS installed, and you want to revert to a release version instead. If you restore in DFU mode, then you can select which IPSW image file to install, download that before putting your Mac into DFU mode, and use that IPSW for the restore. You can also pick which version of Big Sur to use on a bootable installer. However, in the other two methods you get the version that Apple provides. Although this problem appears relatively infrequent, its a serious bug which seems to have affected users for seven months. It needs to be fixed, please. Im very grateful to slice, who took the trouble to explain to me what happened to their M1 MacBook Air just a few days ago.