When running Big Sur 11.1 on an M1 Mac, making a bootable external disk is entirely unpredictable. Some users seem to have all the luck, and the first external SSD they connect to their M1 Mac works fine with the Big Sur installer, and they don’t look back. For others, the macOS installer just can’t do the trick, and a high-quality SSD is left unbootable.
Now that Big Sur can be updated to 11.2, I’ve been looking at how that affects external boot disks, and whether it finally enables them all to work. However, far from 11.2 solving this problem, it actually makes it worse.
Making a new external boot disk
As far as I can tell, macOS 11.2 is no less unreliable at this than was 11.1. Using the 11.2 full installer app, the one SSD (Samsung X5) that worked for me before works still, and the SSD which wouldn’t take a bootable installation of 11.1 still won’t oblige.
The problem occurs at the end of the normal installation phase, when presumably the installer is writing hashes up the Merkle tree, with the installer window claiming that there’s only About a minute remaining. At that stage, Activity Monitor reports that com.apple.MobileSoftwareUpdate.UpdateBrainService is taking lots of CPU, and there’s sustained and intense disk activity for many minutes. When that finally completes, instead of the Mac restarting from the external disk to complete installation, the installer just quits. Trying to restart from the external disk then results in an error.
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