Downloading Mac OS to install on an external SSD

[nuyorican]

The initial message was deleted from this topic on 21 April 2025 at 19:29

[Joseph Ducreux]

Can't you just put the SSD into the iMac, then do an online recovery on it which will install the latest supported OS over the internet and restore whatever you need from Time Machine when you're done?

When you're starting it, hold option+command+R until you see the startup screen.

https://support.apple.com/en-gb/guide/mac-help/mch...

[dogbucket]

You should be able to download the older installer from the app store so it is sitting in your applications folder then make a bootable usb key for that version using the terminal.

Boot the iMac off that then it should be possible to install on the external SSD if that is also plugged in.



https://support.apple.com/en-gb/101578

sudo /Applications/Install\ macOS\ Monterey.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

[dogbucket]

try the link in this page

https://support.apple.com/en-us/102662

I am downloading now on a Sequoia Macbook Pro, although admittedly it is an Intel chipset. Maybe Apple silicon is blocked. If so try a search for a mirror link direct download.

'If downloading an older macOS, you may need to create the bootable installer on an older Mac that is compatible with it.'

Edited by dogbucket on Thursday 17th April 16:34

[mikef]

You can download any MacOS installer including historic versions if you have an Apple Developer account

[Freakuk]

I think the process you have described is correct.

But at boot I'm guessing you would go into the recovery mode and select the new HDD and select that as the primary drive for the OS install.

Do you need anything on the existing HDD, as I guess you could wipe that at the start and force the recovery to use the new drive.

What happens to the old drive assuming you can use the external drive, would performance still be impacted as it would still be communicating running processes on the internal disk?

I don't know the answers but just some thoughts.

[essayer]

I did this with a 2012 iMac which now boots from NVME (via USB)

Fusion/spinning drives are really slow with the later versions of MacOS, might be ok for storage but definitely not to run the OS from

I think I booted the iMac from “internet recovery” Option-command-R on power up then installed latest supported OS

[crowfield]

If the install on the interior drive is good ( just slow because of the drive ) why not just plug in an SSD and clone the existing drive to the new one using Carbon Copy?

[crowfield]

it is an app. https://bombich.com/. will give you a download for a 30 day trial, but I think it is fully functioning , or it was last time I used it. Or you could use the built in disk Utility. https://discussions.apple.com/thread/254472484?sor...

Edited by crowfield on Thursday 17th April 18:39

[Captain_Morgan]

Depending on how proficient with it you are you could install the latest macOS version onto a ssd and boot from that.

Take a look at this and see if your comfortable
https://m.youtube.com/watch?v=xNSOUoYbJdM&vl=e...

[clockworks]

I used to keep a bookable external disk for my iMac, created using Carbon Copy Cloner.

I'm now using an M4 Mac Mini, and from what I've read, you can't boot Macs that use Apple silicon from an external drive.