macOS install/update failing on external SSD (Apple Silicon)

Sparklight wrote:

I mean the data partition for Sequoia, where I have a few days of work in. And yes I get version control, but what I really want to save is the whole setup: installed apps, their logins and settings, etc. Hence the data partition question.

It sounds like you don't have proper backups.

People should always have frequent and regular backups of their computer and all external media (including the cloud) which contains important & unique data.

FYI, There are a lot more new ways to permanently lose access to the data stored on the internal SSD of the recent Macs due to all of the hardware, software, and security changes.

Thanks for the warning on Tahoe.

It definitely has some issues with the recent update patches. Plus in general the new Liquid Glass look is either love it or hate it along with some visual issues with the transparency aspects getting in the way.

If Apple doesn’t support external boot, why don’t I see such a warning at the very top of this article?

Did you make sure not to use the Mac's DFU Port? It can be a bit tricky identifying the correct port as Apple's instructions are a bit confusing.

While I have installed more than one OS on the internal SSD of an M-series Mac, I have never done so on an external SSD. In fact, I've only installed a single copy of macOS onto an external SSD of an Apple Silicon Mac two times.

I can tell you the boot process of the Apple Silicon Macs is complex. When you change to a different boot volume (internal or external), an Apple Silicon Mac will actually need to reboot two times. The first time is to load the "firmware" system files for that specific OS, then reboot while using those "firmware" system files. It also occurs with just a single copy of macOS installed, but it goes unnoticed unless you boot into Recovery Mode or to the Apple Diagnostics. Keep in mind that the internal SSD contains all of these "firmware" system files. So even installing macOS to an external drive will touch some of the hidden system files on the internal SSD. Each OS installation has its own local Recovery Mode and version of Apple Diagnostics. This is where I really began to notice & understand what was happening. That is just the basic boot process.

That two step boot process can also make troubleshooting boot issues very difficult including being able to access the Startup Options or placing a Mac into DFU Mode.

Then you have to contend with the new concept of "ownership" and all of the security aspects of these M-series Macs. These M-series Macs (and even the older T2 Intel Macs) tend to get confused. I have even encountered similar error messages and a simple reboot has sometimes resolved them.....no idea why.

Yes, technically it is possible to install macOS to an external drive, but it isn't as easy or simple as it used to be...*** will definitely always modify the internal SSD. You have to think of the recent Macs as being like a glorified iPad, but without a touch screen. Both the Apple hardware & macOS have been moving in the direction of iPads & iOS. It is hard enough to keep a single macOS installation working these days.

Sparklight wrote:

I mean the data partition for Sequoia, where I have a few days of work in. And yes I get version control, but what I really want to save is the whole setup: installed apps, their logins and settings, etc. Hence the data partition question.

I honestly don't know. Things like apps and user files are easy enough. You should be able to manually mount the partition and copy those out. But when you get into things like logins and settings, it's going to be less reliable.

The problem is the contents of a couple of "Library" folders and some other hidden system folders. Some of that data can be simply copied to another volume and used there without any problem. But some of it can't. And some of it might not even copy at all. Some of the values are unique identifiers that are specific to a single installation. They may not work on a different installation. I've seen this most often with some of these random, less frequently used system settings like Continuity and iCloud. When people setup a new computer or even a user account by copying an old system or Time Machine backup, it never works properly afterwards. Either those unique identifiers don't match the system identifier anymore, or the unique identifier has been copied to a new system and now it's not unique anymore.

You can certainly try to copy it. But if you start to notice strange behaviour later on, this is likely the cause. There's no way to fix this except by recreating everything. And some of these system folders have very strict, funky, undocumented Apple protections on them. You might not be able to copy them at all. And if you do, you might not be able to delete them later.

*Note that none of this applies when setting up a new installation using iCloud. iCloud is designed to clone user data and settings to a new device. This is regularly tested and reliable. But it might not apply to a VM or especially external boot situation. All these warnings about unique identifiers and copying definitely apply to external/multiple boot partitions. It's the great unknown.

And there are some additional options. You can use Disk Utility to try to create an image of the partition and perhaps restore that to a new partition. And there is a low-level command-line tool called "asr" that is designed to backup and restore entire partitions. I don't know if either will work in your case (or at all), but it's something to investigate.

If Apple doesn’t support external boot, why don’t I see such a warning at the very top of this article?

I'm pretty sure I mentioned that during my rant about citing tech support article to me. 😄

Apple has well over a billion users. Most of those users do simple things. They never try anything remotely fancy. And I'm not talking about things like external boot partitions, I mean things that are in the Finder's "File" menu or context menu. I mean first-order Apple apps that have subscription options. Apple regularly breaks these things. On the rare occasions that something does get fixed, the timeframe is always in months - always.

It's a trial and error learning process. You learned a lesson this time. I learned a similar lesson just the other day. I figured Tahoe was relatively stable and it would be safe to update to 26.4. I was wrong. So it's taken me a few days to roll back my machine to 26.3.1. I'm a developer myself, so it's a different perspective than most people. My suggestion is weight the trade offs between how much time you're going to spend trying to fix it. Maybe it's not really worth it. And be much more cautious going forward. For things like new device support, there are other, much less risky options. Just yesterday I downloaded Xcode 26.5, installed only the device support runtimes for my macOS 26.5 VM, then trashed that version of Xcode. You can do something similar with iOS devices.

Sparklight wrote:

The latest: I created a new clean partition (hard partition, not APFS volume)

FYI, it is not a good idea to create multiple partitions on a drive. People almost always realize later on that one or more partitions is too small. There is no easy way to fix the problem.

With the APFS file system there is absolutely no reason for multiple partitions. Create new APFS volumes which act like a partition without the need to limit them. Adding & deleting an APFS volume is simple & easy.

and installed 15.7.5 to it. After the install completes, I find myself back at my primary OS, not in 15.7.5. And when I go to Startup Disk and select it, it selects, it shows 15.7.5, but then at the last minute when I tell it to reboot to the new disk I get:

Unable to set startup disk

An error occurred while setting “NewOS” as the startup disk: The operation couldn’t be completed. (SDErrorDomain error 108.)

Does this shed light on anything?

Unfortunately Apple doesn't provide information about most error codes. I was able to find two posts online where a person found the source of their problem:

• • Bad or Incompatible hub or enclosure
  • Used the Mac's DFU Port for the external drive

Sparklight wrote:

Is there a well known bug here?

Define "well-known". I've known about it for years and tell people whenever they ask. But usually they only ask after-the-fact. And in other cases they simply don't believe me.

Is there a well-known fix?

No. Apple doesn't support external boot devices, or multiple boot partitions, and hasn't done so for many years.

But... but... but... "worked fine" you said. Of course. In many cases, it doesn't work at all. In cases where it does work for a while, it will fail eventually if you ever try to update or anything, which people are absolutely addicted to. You can cite those Apple support articles and tell me how much Apple supports this process until you're blue in the face. It ain't never gonna make that thing boot.

How often do you buy a new phone anyway? This is one of those "first world problems", eh? My iPhone 11 Pro is still running fine.

Looks like I may have to wipe that disk and start over.

That's what I would recommend.

If so, is there a way to save my data partition, so I don't have to reinstall everything all over again? There's two or three days of work in that partition, I'd really rather not!

I'm confused a bit. Which data partition are you talking about. You should have 3, no? Please tell me you weren't trying to use a single data partition with all of them. That would be very, very bad.

If you're developing, then you should have everything in a git repository somewhere. If not, then you should be able to mount those other partitions and extract the data from them, even if they won't boot.

For the record, I would recommend using Sequoia as primary boot version. It's stable. It runs Xcode 26. As long as you don't buy any new phones soon, you should be fine. You can always just use one of the older phones for debugging. Please tell me you're keeping your old phones and keeping them on an old OS version.

I never ran Sonoma except for testing. It wasn't Apple's best work. But it was sure better than Tahoe. The latest 26.4 is easily the slowest, least stable version of Tahoe, including the first beta. I rolled back to 26.3.1 just so I could use the News+ subscription I pay for.

If you did want to try new OS versions, I recommend a virtual machine. Don't use it for development though. Find a stable version and break the upgrade addiction.

FYI, the web site of The Eclectic Light Company has a number of articles about external booting on Apple Silicon Macs. The articles on that site seem to explain things in much more detail than Apple's Support articles do.

Scroll way down to the "External boot disks" section of The Eclectic Light Company – M-series Macs, where there are links to more than a couple of dozen articles on the subject.

HWTech wrote:

With the APFS file system there is absolutely no reason for multiple partitions.

FWIW just the other day I had to create a small 100 GB Mac OS Extended partition to an otherwise 1500 GB APFS disk so I could use CCC to make a bootable El Capitan backup to it.

El Capitan can boot to it and it just ignores APFS volumes having Big Sur, Monterey and Ventura CCC bootable backups. ...yes, I have all those MacBook* backups on a single HDD but there no important data there -- the purpose is to easily restore a bootable old macOS and bypass a lengthy install.