Before we get to the bad news, I thought I should share the good: it is possible to turn the majority of external SSDs into bootable disks for M1 Macs. Thanks to various users who’ve tried this, I can now reproducibly install a bootable copy of Big Sur 11.2 on a USB-C SSD which had previously refused to complete macOS installation. The secret is to avoid connecting the disk to your M1 Mac’s USB-C port. I know, why would anyone want to connect a USB-C disk to a USB-C port, you wonder.
Creating an external bootable disk for an M1 Mac still involves trial and error, though. If the disk has a Thunderbolt interface, then you should be able to get it to work reliably using a certified Thunderbolt 3 cable to one of the USB-C/TB4 ports on your M1. But most external SSDs use cheaper SATA and USB-C interfaces. Some of those will work properly when connected to a USB-C/TB4 port, but many will fail to complete the Big Sur installation. In those cases, connect them instead to one of the USB-A ports on your M1 Mac mini. Whether this works through a dock to a USB-C/TB4 port on an M1 MacBook Air or Pro, I’m afraid I don’t know.
You wouldn’t want to leave your external SSD connected to a USB-A port any longer than you have to, though. Whilst I was about it, I used my disk benchmarking app Stibium to see how big a performance hit USB-A is against USB-C. My Samsung 860 EVO SSD running through an unbranded SATA to USB-C case read at around 400 MB/s and wrote at about 390 MB/s. Move it to a USB-A port and those figures fell to about 300 and 310 MB/s respectively – a far cry from M1 internal storage which should be turning in around 3 GB/s, or an external Samsung X5 at better than 2 GB/s.
So once you’ve got Big Sur installed successfully on your external disk, you’ll therefore want to move it from USB-A to USB-C as soon as you can.
The bad news is that, once you’ve created your external bootable disk with Big Sur 11.2, you probably won’t be able to update it to 11.2.1. That’s not apparently related to the initial problem of installing 11.2, and switching it to a different port proves to be of no help here. It’s because the 11.2.1 update is currently broken.
I have two M1 Macs, a Mac mini and a MacBook Pro. Both updated their internal storage flawlessly to 11.2.1 through my local Content Caching Server. I also now have two external bootable SSDs which are stuck with macOS 11.2 and can’t be updated to 11.2.1. As Apple hasn’t yet released a full updater for 11.2.1, which in itself suggests that there’s a problem, there seems no workaround until Apple fixes this.
The most infuriating point about the failure of the 11.2.1 update is that it occurs at the last moment before the update should be installed: in order for that to begin, on an M1 Mac (but not Intel models), the user has to enter their password. That’s when it falls apart, because no matter what you enter, the installer refuses to accept it, shaking its dialog and refusing to progress every single time. This happens whether the keyboard is running wirelessly, or connected to a USB port. Sometimes, when in wireless mode, the password entry box runs away streaming in characters which you haven’t typed. At other times it seems to behave until it yet again vigorously shakes its disapproval at what you know is the correct password.
Because of another bug in the 11.2 installation process, which leads to the copying of the existing user as well as creation of a new account, I’ve tried updating from each of those two accounts, with identical failures.
The workaround which I tested when trying to install 11.2 was to run the full Big Sur installer. Although Apple released the 11.2.1 update on 9 February 2021 and normally makes its full installer available quickly, that available in the App Store is still apparently 11.2, and listing available full installers using the command
only offers one Big Sur installer,
* Title: macOS Big Sur, Version: 11.2, Size: 12208832027K
and 11.2.1 is nowhere to be seen.
Without a full installer, it’s impossible to create a bootable installer containing 11.2.1. Apple has made an IPSW file available, which can be used to restore the internal storage of an M1 Mac via Configurator, but can’t be used on an external disk. Without a standalone updater, there’s simply no alternative to the online update.
So why hasn’t Apple told us of these problems updating to 11.2.1? Does Apple test its updaters on external macOS installations? If it does, and has discovered this problem, why has no one informed users?