External Boot Drive

I am currently running Sonoma on a brand new iMac. I need to create an external boot drive for an old computer using Mohave as the OS. Sonoma will not allow me to create an external drive with anything older than the running OS. Any ideas on how to accomplish this?

Has links to older OS’s on the App Store (that you can’t seem to find any other way). It lists back to High Sierra.

Good Luck!

But you may not be able to run the installer for a system too old for your hardware. You might need to run it from within a VM or using an old Mac.

But that having been said, if you have an installer application (via one of those download links), once you run it, it should ask where you want to install the system. So specify your external drive and it should work.

How about creating a bootable macOS installer with Mohave on an external hard drive or thumb drive?

You can then install Mohave on another hard drive or on the computer itself.

This Apple website has the details:

All of Apple’s support docs don’t help if you have a machine that can’t install Mojave, because the AppStore won’t let you download it.

Apple has a web site with downloadable installers for El Capitan, Sierra, and I think Mavericks and Yosemite, but that just gives you and “install os.dmg” file that doesn’t expand to include the terminal “createmedia” executable.

I think I’ve still found a way to get a Sierra installer, but I need to test it and will type it up later if I figure it out. From there you could upgrade to Mojave. If I can’t figure it out, I do have the Mojave installer that I can share a Dropbox link and from that you can follow Apple’s instructions to create a bootable flash drive.

If the other machine has a Recovery partition, that might be a lot easier. Start the machine with cmd-shift-option R and try to install the oldest MacOS version from the internet. But I’ll keep working on trying to make a bootable installer for Sierra and provide instructions.

Yes. You need to jump through a few more hoops.

According to an Apple discussion thread, that DMG is a disk image. Inside it is an installer package. If you run that package, it doesn’t try to install Mac OS X, but it creates the installer application that you would run to perform the installation. (If it auto-runs, you can quit it, leaving the app behind for you to work with.)

You can then look within that package to find the “createinstallmedia” utility to making bootable media.

For the oldest installers, which don’t have “createinstallmedia”, you can look inside to find a “InstallESD.dmg” disk image. You can burn this image to a dual-layer DVD to make an install DVD. I’m not sure if/how you can make a bootable flash drive from this image.

But there’s one massive problem. You can’t run the package file to make the installer application if your Mac is incompatible with that version of Mac OS X. So you’ll need an old Mac (or maybe a suitable VM) to perform that part of the procedure.

I’ve done all that. There is no createinstallmedia - there is just InstallESD.dmg, which, as you say,

You can burn this image to a dual-layer DVD to make an install DVD. I’m not sure if/how you can make a bootable flash drive from this image.

But there’s one massive problem. You can’t run the package file to make the installer application if your Mac is incompatible with that version of Mac OS X. So you’ll need an old Mac (or maybe a suitable VM) to perform that part of the procedure.

So instead I am just uploading the Mojave installer to iCloud (which I stored on an older machine two years ago and saved) and as soon as it finishes I’ll share a link.

Yes. For the older installers (Mountain Lion and older, I believe), that will be the case.

I found this article:

Which describes how you can make a bootable flash drive from that InstallESD.dmg file. It’s a bit of work (partition your flash drive, then use Disk Utility to “restore” the dmg file to the drive), but worth a try.

Mojave has “createinstallmedia”. So you can make a bootable drive for that.

$ /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --help
Usage: createinstallmedia --volume <path to volume to convert>

Arguments
--volume, A path to a volume that can be unmounted and erased to create the install media.
--nointeraction, Erase the disk pointed to by volume without prompting for confirmation.
--downloadassets, Download on-demand assets that may be required for installation.

Example: createinstallmedia --volume /Volumes/Untitled

This tool must be run as root.

But annoyingly, I can’t delete the installer, having downloaded it. Its internal “InstallESD.dmg” file is protected by SIP for some brain-dead reason. So now I’ve got to go reboot into recovery mode to delete the installer I just downloaded. :confounded:

(Update: Fortunately, I didn’t need to disable SIP. I could just open a shell and delete the file while in Recovery mode. But still a completely unnecessary waste of time.)

Ok, here is a downloadable link to the Mojave installer: Proton Drive

Hopefully that works out. Apple has instructions on how to create a bootable installer here: Create a bootable installer - Apple Support

You will probably need to alter the path to the createinstallmedia command, based on where you downloaded the link to, and to the actual location of the USB flash drive you will install to (Apple gives the example /Volumes/MyVolume but before you run the command you can find out yourself in Terminal by typing the command “ls /Volumes” to get the actual name.) MacWorld also details exactly how to prepare the flash drive here: How to create a bootable USB macOS installer | Macworld

Good luck, and I hope that works out for you. I’ll probably delete the file in about a week. (It’s not anything proprietary, but at least you’ll be able to download it.)

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This thread reminded me of how Open Core Legacy Patcher downloads OS systems back to Big Sur but nothing further. Then, I came across this article which explains how to use dosdude’s Mojave Patcher app to get the full installer so that is another choice in case someone needs it later:

I tried the program on Sonoma (Intel) but not sure if it works on Apple Silicon. You only need to use the download function as the article explains.