External SSD. Crucial X9. Installed Sequoia can’t be used as a Startup Disk

The message usually means the wrong version of Sequoia was installed. That is, an Intel build rather than for Apple Silicon.

I had the same thing happen with Sonoma when I tried to use the downloaded installer I used on a 2018 Intel mini for an M2 Pro mini. It installed, but then wouldn't start up to the external. The M2 mini gave me the same message. I had to download Sonoma again from the M2 and then install that to the external drive.

inknpaper wrote:

1. Thanks for your reply :) Would it be possible to...

Create a sparsebundle and then deploy it on the external SSD?

No.

2. Create a bootable disk using the Terminal and then attempt to install OSX on the SSD?

Should work, but so should Recovery Mode.

...or wouldn't either of them work because of how you described it; no matter what anyone does, the Crucial X9 (or any other SSD) won't work as an external and independent OSX on the MacBook Air M1 (and others alike)?

You can still have a bootable macOS boot drive with M-series Macs, but there are more restrictions such as not being able to use that boot drive with other Macs (especially Intel Macs).

What a shame because I have been using externals (and sparsebundles) for many years.

I'm convinced there is a way :)

Make sure you erase the whole physical external drive as GUID partition and APFS (top option). Some external drives may come with odd partition & file system layouts on them from the factory. Sometimes the manufacturer even places hidden items in areas of the drive that can wreak havoc with certain systems. This is why any new drive should be partitioned & formatted (macOS calls this process "erasing" in Disk Utility) so it has a partition & file system layout compatible with the OS using the drive.

Disk Utility now hides the physical drives & APFS Containers by default now. Within Disk Utility click "Show" and select "Show All Devices" so that the physical drives and hidden APFS Containers appear on the left pane of Disk Utility. Do not erase the whole physical internal SSD of an M-series Mac or you will need another Mac to perform a DFU firmware Restore. Here is an Apple article showing the process of erasing the whole physical external drive:

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

Make sure to connect the external drive directly to the Mac and disconnect all other external devices in case one of them is causing a problem. Try using another USB-C port as well and maybe even a different cable. If you are using an adapter, then maybe try using a different adapter. It is always possible the external drive has a problem or maybe there is even an issue with the laptop's USB-C ports.

If you are connecting the external drive directly to the Mac, then perhaps try connecting it to a powered USB3 hub which can sometimes act as a buffer in case of any USB related issues (I had an old Intel Mac which had issues with USB3 external drives unless I used a powered USB3 hub) and to provide extra power to the external drive.

inknpaper wrote:

...It doesn't work. I've tried twice now. The first attempt was APFS only and the second was APFS (Case-sensitive). I'm really at a loss.

Do NOT use the Case-Sensitive option or you will have problems. Very few macOS users should ever be using the Case-Sensitive option and then only for very specific reasons & circumstances that require it.

The partitioned part works fine with Time Machine.

Are you trying to make a bootable macOS boot drive out of your TM backup drive? Don't do it. Never put your backups at risk.

Plus this could be the problem. TM backup drives are different than a standard external data drive.

Use another drive for your external macOS boot drive.

The drive is fine.

You don't know that for sure. I've seen things over the years that were not obvious at first sight (or second, or third....).

You can only use an external macOS M-series Mac boot drive on a single M-series Mac due to the new concept of "Ownership" (I don't know whether it would work if both M-series Macs had the same "Owner" or not as I have not had time to experiment with my organization's Macs). You also cannot take a macOS external boot drive made for an M-series Mac and use it on an Intel Mac, or the other way around.

Plus, all external macOS boot drives on an M-series Mac still requires a working internal macOS boot drive even to boot an external boot drive (or even just to access the Startup Options screen).

FYI, think of these Mac computers as being glorified iPads without a built-in touchscreen because that is where Apple has been going with the Apple computers for some years now both with the hardware & software. I foresee bootable external drives as disappearing in the not-too distant future.

Hmm, with only an M1 Mac on hand, you should already be getting only the correct version.

Have you installed an OS to this external drive before? If not, you have to start up to Recovery Mode and define the external drive as safe with your admin password. Otherwise, it will not be bootable no matter what you do.

It's been a while since I installed macOS on my external, so I have to guess a bit.

You start into Recovery Mode and choose to install macOS. It defaults to the internal drive, but then use the option to choose the external. At that point, it should ask for your admin password to trust the chosen drive.

In fact, I wasn't even aware that there are different installers for different machines and external SSDs.

I didn't either until I ran into that situation. I figured it would be a universal build and install according to the type of Mac you have.

how did you get what you needed, your M2?

I downloaded Sonoma (and really, any major OS release) through the App Store. That way the OS can detect what hardware you have and download the appropriate version.