How to download macOS Sonoma installer to External HD from App Store?

Paul Downie wrote:

I’m rapidly running out of space on my 2023 Mac mini: and thinking of installing the OS onto an external drive.

I don’t have the space on my main drive for the installer.

How can I put it onto an external drive?

The App Store downloads macOS installers to your Applications folder. I don't think there is any way to redirect the download to an alternate location. You will have to at least temporarly free up enough space on your mini's drive to download the installer.

Have you considered moving your data to the external drive instead of creating an external boot drive? Things like the Photos, Music & TV libraries can be moved easily enough. Most other data can also be relocated. But unfortunately the Outlook database and perhaps the Mail database cannot be relocated.

You can boot into Recovery Mode to access the online macOS installer.

How to reinstall macOS - Apple Support

Just make sure when you erase the external SSD that you select the whole physical SSD and erase it as GUID partition and APFS (top option). Within Disk Utility you may need to click "View" and select "Show All Devices" before the physical drives appear on the left pane of Disk Utility. If you do not erase the whole physical external drive, then you may encounter issues since some external drives ship with odd partition layouts.

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

The solution is not to install macOS on an external drive but to move your data to an external drive. Things that suck up tons of space on your drive like music, movies, TV shows, photos can easily be moved to an external drive and there is no reason for them to be required to be on your boot drive:

Change where your music files are stored on Mac - Apple Support

You can simply move your entire Photos library to an external drive and hold down the shift key the next time you open Photos and select the new location.

You can also move any of other data to the external drive as well.

Once satisfied that all the stuff on the external is accessible, delete the items on your internal drive and viola! more space.

Here are some related instructions: Use an external SSD as your startup disk … - Apple Community

Migrate just a subset of the current contents.

Or probably better in this situation, start over with nothing installed, and manually install and manually copy over what you need. Copy the old data to some external storage device somewhere, and pull over from that what you need.

Or get a Mac better sized for your usage.

When you click on the file to open it you will then receive a message asking where you want to install the OS. Select the icon for the external SSD. If it doesn't show up you may have to first reformat it. If so, only use Disk Utility. Maintenance apps that come with disks always cause problems for Macs and should be deleted and not used.

I'm having this same issue. I have a 512 GB iMac migrating from a 2TB iMac (ridiculous I know but that's life) and I just can't figure out how to get everything I need on the smaller iMac. I'm trying to install Sonoma on a Samsung T9 SSD (2TB) and I can't seem to download Sonoma from the App Store. It immediately tries to install and gives an error and then it is gone.

How about Mail, Messages, Libraries, and other huge stuff like that? My user folder is 75GB alone and I've off loaded as much as I can. Applications folder as well. I have the older iMac down to 468GB on the HD and It won't migrate over.