Installing Sonoma on an external drive

sashasdad1 wrote:

I was using a Crucial X9 external HD as the destination for Sonoma, since returned as I was told that wouldn't work. I have some software that needs that OS and I wanted to put that on there as well.

I have iMac 24 inch, M1, 2021, 16GB Memory, Tahoe 26.1

Thanks for these details.

I have 4 USB ports on the back of the iMac on the left side as you look at it. I was using the 3rd from the left as you look at it. It has a lightening shape symbol above it and the 4th one.

You cannot use the 4th port from the left for the destination drive while installing macOS to an external drive. That is the special DFU Port. This Apple article (contained within an Apple article I linked twice previously) has instructions for identifying the DFU Port which should not be used while installing macOS to an external drive.

How to identify the DFU port on Mac - Apple Support

Once macOS has been fully installed to the external drive, then any of the USB-C ports can be used for booting a macOS USB drive.

I did not have an USB installer stick. That was never mentioned to me. I don't know what that is. Do I need one? I just had the Crucial X9 HD I as trying to install Sonoma on.

And now we come to the critical little detail that was needed to solve your problem. You need two physical drives here....one for the USB installer and another one for the full macOS installation. Technically you can use a single SSD here, but it complicates things and could leave you with 20GB of storage that cannot be merged into the macOS volume when all is finished (depends how the drive is partitioned, but even when done correctly it involves more work that may or may not require using the command line which is always risky since there are no safety nets).

Apple's instructions are poorly written in some ways. However, the Apple article I linked in my first reply to you for "using an external storage device as a Mac startup disk" mentions that the destination volume for the macOS installation must be an APFS volume and you may need to use Disk Utility to erase that volume first. I've made the lines relevant for your iMac in bold in the following quote from that Apple article:

from How to use an external storage device as a Mac startup disk - Apple Support :

Check the format of the storage device

A Mac startup disk requires either of these file system formats:

* APFS format is recommended for macOS High Sierra or later, especially if your storage device is a solid-state drive (SSD).

* Mac OS Extended (Journaled) format, also known as HFS+, is also acceptable for traditional hard drives, and it's required for macOS Sierra or earlier.

You can use Disk Utility to select your storage device and see which format it's using. You can also use Disk Utility to erase it using either format.

The volume for your macOS Sonoma destination must be APFS. You cannot erase the volume/partition containing the installer since you are booted from it.

Get a 20GB+ USB stick for your bootable macOS Sonoma USB installer. And erase your Crucial SSD as GUID partition and APFS (top option). You should then be able to install macOS Sonoma onto the Crucial SSD, just make sure to avoid using the right-most USB-C port during the install process.

If you still have issues, then try using the 1st or 2nd USB-C port instead for your Crucial SSD while you are installing Sonoma....just in case there is some issue with a USB3 SSD using a USB4 port.

Since I seem to have gotten some bad info from Geek Squad let me ask... I was told a solid state external HD like the Crucial would work better with an OS on it than a My Passport would. Is that true?

An SSD is needed for an external boot drive. A traditional spinning Hard Drive is just too slow to be useful these days for booting macOS.

Plus Macs are very picky about the drives used for booting macOS. Perhaps the tech is aware of more problems booting macOS from a WD MyPassport drive than a Crucial SSD. I don't have experience using pre-built consumer external drives for booting macOS, but either one should work as long as they are at least a USB3 SSD capable of least 500MB/s transfer rates.

Thank you for your patience. Technology mostly baffles me. :)

It baffles me at times as well.

sashasdad1 wrote:

Those instructions mention the USB stick must be erased as GUID partition and MacOS Extended (Journaled).

I eventually did this and was successful. In the following instructions I get to #4 and then it won't work.

4. Select the bootable installer, then click Continue. 

When I clicked on the external drive icon (bootable installer) to select it there is a message that it can't open because it isn't APFS.

So you get this error message immediately after selecting the USB installer? Or does it occur once the macOS installer has booted?

If it won't let you select the USB installer, then perhaps the issue is with the USB stick you are using. Macs can be very picky about the drives used for booting, plus most USB sticks are pure junk sometimes even from a known good brand. Try using another good quality USB stick for the bootable USB installer such as from SanDisk....I've also had good luck with the MicroCenter USB sticks.

Keep in mind we need very detailed information on exactly when the problem occurs since we are not there to see it.

A question we should have asked originally is what is the exact model of your iMac? You can get this information by clicking the Apple menu and selecting "About This Mac".

According to Geek Squad installing an Apple OS on an external drive is not possible. Apple won't let one do that. It has to be an internal drive.

That tech obviously is not very familiar with Macs as others have already noted. Here is an Apple article which I linked earlier that contradicts them:

How to use an external storage device as a Mac startup disk - Apple Support

Which brings me to my next question......which USB port on your iMac is the destination drive for macOS Sonoma connected? You must be very specific here. Describe the port location while looking at the back of the iMac. And which USB port is the USB installer stick connected?

sashasdad1 wrote:

Should the USB stick be connected as well as the Crucial HD?

you only need to connect the USB stick to create the bootable installer. here's the link again on how to create the bootable installer>> Create a bootable installer for macOS - Apple Support. follow ALL of the steps. have that page open on another computer if possible. and once you have created the bootable installer, you can then connect both the USB stick AND the crucial drive to install Sonoma on the external crucial drive.

The directions mention "storage device". Is that the USB stick or the Crucial HD?

both the thumb drive and crucial drive are storage devices. but your post says "crucial HD". if it is a rotational HDD, then as already mentioned, it really isn't suited for use as a boot drive. you would want to source an external SSD. I have had good luck with Samsung T7 SSDs

Am I supposed to download Sonoma from the internet onto the USB stick rather than my computer's HD? It is currently in my Applications Folder.

that is good. the Sonoma installer NEEDS to be in your applications folder to create your bootable installer.

I guess I am unsure of the purpose of the USB stick.

once you've read the article Create a bootable installer for macOS then hopefully it will become clear as to the purpose of the USB stick. near the end of that article, you are told how to use the bootable installer after it has been created.

Thank you!

you're welcome.

sashasdad1 wrote:

1. I'm trying to install Sonoma onto a Crucial X9 external HD. My computer is an iMac, Tahoe 26.1 OS.

If this is an M-series Mac, then you must make sure not to use the DFU Port for the destination drive when installing the full macOS on an external drive. See the following Apple article for details (it includes a link to another Apple article for identifying which USB-C port is the DFU Port):

How to use an external storage device as a Mac startup disk - Apple Support

All is well until here.

Configure the erase options:

Name:•  Enter a name for the drive (e.g., "Sonoma External").

Format: Choose APFS•  (Apple File System).

Scheme: Choose GUID Partition Map• .

Click Erase and then Done1.  when the process is complete. 

A choice for Scheme never appeared.

I don't understand "Scheme". What instructions are you following?

Step 2 works.

Step 2 of what instructions?

When I do this:

Step 3: Install macOS Sonoma to the External Drive 

Launch the installer1.  again from your Applications folder.

I get "To install this version of macOS, please see Create a bootable installer for macOS - Apple Support

You cannot run a macOS installer app for an older OS which is why you must use a bootable macOS USB installer.

I follow those directions in Terminal successfully until I get a message "APFS disks may not be used as bootable install media. An error occurred erasing the disk."

Re-read the instructions in that Apple article for creating a bootable macOS USB installer. Those instructions mention the USB stick must be erased as GUID partition and MacOS Extended (Journaled). I know, it is crazy that the command line utility you use to create the USB installer will erase it again anyway. It's Apple.

sashasdad1 wrote:

This is beginning to make sense to me. I was trying to use the Crucial external HD as a bootable installer which didn't work. The Crucial HD is solid state so that should be ok.

I would assume that would work fine. crucial makes decent products, and since it's solid-state, you should be good to go.

So once I open the bootable installer (flash drive) I can use it to install Sonoma on the Crucial HD?

yes. just follow the "how to use" section of Create a bootable installer for macOS. it gives you the steps you need to use the bootable installer.

I was reading two sets of instructions for what I didn't realize was two different activities....

understandable. some of this SEEMS hard the first time it is tried. but it's kind of like riding a bike. once you learn it, you should never forget it. :)

Thank you!

you're welcome.

sashasdad1 wrote:

According to Geek Squad installing an Apple OS on an external drive is not possible. Apple won't let one do that. It has to be an internal drive.

then that particular geek squad helper is incorrect. not even geek squad employees are infallible. if you follow the instructions provided by HWTech to the letter, you will be running in no time.

sashasdad1 wrote:

1. I'm trying to install Sonoma onto a Crucial X9 external HD. My computer is an iMac, Tahoe 26.1 OS.

Why? In most cases, you would be better off running Sonoma in a VM like VirtualBuddy.