External drive not showing in Startup Disk in System Preferences

My 2019 iMAC using MACOS 10,14.6 (Mojave) and Macbook Pro 2019 also with Mojave will not show an external drive with Monterey as a choice in the startup disk in system preferences even though it shows on desktop.
The only way I can get Monterey to be the startup disk is to shut down and restart holding down the option key.

I suspect the culprit is SMART status: not supported. How do I get SMART to be supported?

See the related advice here:

However your issue of the Monterey start-up disk not appearing is sys-prefs is not promising!

Thanks. The drive is good as per Apple support. They are researching how to get it show in system preferences startup disk.

Michael Paine mpainesyd
December 29

See the related advice here:

Is my Time Machine drive causing issues? TidBITS Talk

Since you mentioned you have been using it for several years I’m wondering if the drive is starting to fail. Why don’t you try another drive and see if you still have the problem.

However your issue of the Monterey start-up disk not appearing is sys-prefs is not promising!

This one is new to me.

If you couldn’t boot from the drive at all, I would ask you to check to make sure your T2 chip is configured to permit booting external drives. But you can boot it (via the option key), so that’s not an issue.

I suspect that it’s a problem with Monterey simply being too new for the Mojave Startup Disk preference panel to recognize. In order to detect bootable volumes, it looks for particular files/directories on the volume. If it doesn’t see what it’s looking for, it will assume the volume to be non-bootable and won’t show it.

In this case, Mojave boots from a single APFS disk volume. I assume its Startup Disk panel will be looking for a single volume containing boot code (boot loader, kernel, other key files, etc.)

Starting with macOS Catalina, however, the OS was split into two different APFS volumes - a System volume (which is a cryptographically sealed snapshot of itself since Big Sur) and a Data volume, which are bound to each other using the APFS Volume Group mechanism. Startup Disk would need to recognize this, look for key files in each of the two volumes, and look for the volume group. Since Mojave doesn’t use this mechanism, it probably doesn’t recognize it, and so won’t show the Monterey system as bootable.

At least that’s my theory at this time.

As for SMART status, macOS does not include the device drivers necessary to check SMART status on USB drives. You need to add third-party software if you want to read it. As far as I know, the only third-party driver to support this is the open source OS-X-SAT-SMART driver.

If you don’t want to manually compile and install it yourself, Binary Fruit ships a signed build of it with their DriveDx product. If you want to download and install their build independently of DriveDx, see their page describing USB support.

Even with the SAT-SMART driver, your USB drive/enclosure will still need to support SMART data - not all do. But without the driver, macOS won’t even try to read it.

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The original SAT-SMART driver on GitHub has pretty much been abandoned for 7 years now because the developer could not afford to pay for an AppleID to sign and/or notarize it. So you really do need to go to either the DriveDx site mentioned or use SMART Utility or one of a few others that has built the SAT-SMART driver into their product to get a signed version that will load in modern macOS’s.

Yes. But it’s worth noting that BinaryFruit’s fork is identical. So either they aren’t pushing their changes back to GitHub or the code has remained compatible after all these years (not counting use of new compliers and tools to support new platforms).

I wouldn’t suggest using the GitHub code unless you are willing and able to compile it yourself (and sign it with your own Apple ID), which is far more effort than its worth unless you plan on updating the code.

Yes, I’ve been following this from even before signatures were required and have helped a few users deal with it through multiple macOS upgrades and for the most part it works for a most USB externals. It’s continued to work for me with all the USB drives I still have. Some had to fall back to the 0.8 version and a very small number were able to modify the code to accommodate a new drive, but that’s really complicated. Then there are a few more who’s drive does not recognize the driver at all an either didn’t have the skills to modify the code or the drive was just beyond the scope of any version of the driver, so have to do without SMART status.

Mojave Startup Disk System Preference does not recognize Big Sur or Monterey volumes as bootable, but does recognize Catalina volumes. Option key on restart is the way to do it. Catalina Startup Disk does recognize Big Sur and Monterey volumes. Booted from external Monterey, you should be able to select it as Startup Disk.


Thanks. This seems to support my theory.

Catalina introduced splitting the boot volume into two separate system/data volumes. Big Sur introduced the signed/sealed snapshot of the system volume as the boot device. So it makes sense that Catalina will recognize subsequent OS versions.

I’m a bit surprised that Mojave recognizes Catalina, but maybe the System volume resembles the older single-boot-volume configuration enough to be recognized.

Or some Mojave update added support for Catalina. The last Mojave update, 10.14.6, was released in July 2021. Catalina was released in October 2019. So there was plenty of time to patch the Startup Disk preference panel.

But if Apple updated Mojave to add support, then I would have expected them to patch it for Big Sur support as well, since it was released in November 2020 - well before the last Mojave update shipped.

Mojave as far back as 10.14.3 recognizes Catalina drives for startup disks. Mojave will show two separate disk icons in the Finder, one for the System and the other Data. Since I can’t update past Catalina, I can’t say whether Mojave recognizes anything newer since I run Monterey in a VM if needed.

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Thanks again. I am still working with Apple’s senior level. We believe the last attempt will be to take an external drive and make a bootable copy of Catalina, then update it to Big Sur, and then update to Monterey. Other methods did not work such as writing code into terminal or going through recovery to install a Monterey system.
I am surprised Apple has made the Shut down-Power on + Option the only way. It is like working with Boot Camp instead of Parallels.

There have been lots of changes in the boot processes for Macs as you go through the journey to Monterey and M1 Macs. You might want to take a look at some of the articles that Howard Oakley has published over at eclecticlight.co which are quite enlightening.

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As I stated, I can’t go past Catalina and don’t need to upgrade anyway. My post was in response to David regarding what Mojave sees for startup disks.

Understood. It’s just that Howard’s articles are very interesting and well researched on the topic of how the Mac boot process has changed over the last few releases.

I think I may have seen some of his articles before when researching firmware updates but thanks for posting the link.

I have been told that Monterey needs to be on a Thunderbolt SSD to have any chance of booting externally. Is yours external drive a Thunderbolt SSD? Making a bootable back is no longer as easy as it used to be. I recently read in Take Control of Backing Up Your Mac that making a bootable back is not only much more difficult, but it is now optional because of the recovery mode.

Yes it is a thunderbolt SSD (Samsung T5 with USB-C thunderbolt connection on both ends).
I have been working with Apple for 3 sessioins-each of 4 hrs. or more. It is bootable from shut down, then startup holding Option. It just does not show up in systems preferences’ startup disk when running Mojave (10.14.6).

This is the first time I’ve heard about this. Please provide a link, if you have one.

Making a bootable backup, since Big Sur, has become quite difficult.

It can only be done using an internal Apple utility (ASR - Apple Software Restore). Unfortunately, it is only capable of cloning entire volumes and has no facility for incremental backups (of just files changed since a previous backup). It also has some problems (especially on M1-based Macs) that prevent it from correctly making bootable backups.

Those software tools that are able to make bootable backups of Big Sur (and Monterey, if it’s even possible) use ASR under the covers in order to make a bootable backup. But because of the way ASR works, using it will wipe all previous backups from the target volume, so it should only be used for the initial backup. Subsequent backups must use other (more traditional) backup mechanisms, which can not update the sealed System volume.

In general, it is not recommended that you make bootable backups of Mac systems post-Big Sur. it is instead recommended that you only backup your Data volume. When you need to restore from your backup, use recovery mode (or Internet recovery, if necessary) to reinstall macOS and then use Migration Assistant to restore your apps and data from your backup.

All this having been said, I did make a bootable Big Sur backup from my 2018 (Intel) Mac mini. The result is bootable, but booting from that backup (which is a USB hard drive) is painfully slow. Not something I would want to use except in a true emergency situation. My subsequent incremental backups only back up the data partition.

I could, of course, make another System backup (wiping my snapshots that have older backups) to update the backed up System volume, but I don’t want to do that. I could also boot the backup and run System Update to update its System volume, but booting from the HDD is so slow I don’t think it’s worth bothering. In the future, I’m probably going to forget bootable backups altogether - they’re too inconvenient right now, and I see no sign of Apple trying to make it easier.

Monterey is bootable from external USB drive on Intel Macs. External Thunderbolt (not just USB-C) drive required for M1 Macs.

Startup Disk Preference Panel works the same on Mojave, High Sierra and Sierra. Catalina appears, Big Sur and Monterey do not.

This is incorrect. My M1 Mac on Monterrey can boot just fine from an external USB-attached drive. Even HDDs work, it just takes forever.

Here’s some instructions for making a bootable external USB-C disk (SATA via USB-C actually) on an M1 in Monterey courtesy of Howard Oakley.