Installing Sonoma on an external drive

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.