How to make a bootable USB copy of Mojave downloaded from Apple when the file is an app and not a disk image

big surbootable-diskmojave

When I try to open the Mojave install app on my computer with Big Sur, it says that the operating system is too old and so it will not let me open it in order to install it on the external usb drive.

Additionally, the alternative method – restoring Mojave onto the external drive is not possible because the Mojave install file is an app and not a disk image.

How can I proceed to create a bootable copy of Mojave on my external USB drive considering I am on a computer with Big Sur and the Mojave download is an app file which will not open and it is not a DMG file which I would otherwise be able to restore onto the external USB?

Best Answer

Follow the createinstallmedia directions here: How to create a bootable installer for macOS

For the sake of completeness, here’s the command you would run in Terminal to create the install media for Mojave (it’s slightly different for each OS version):

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

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Creating bootable USB for OS X Leopard

I've done a bit of searching, and found these two articles:

The first article was written with Leopard in mind and does advocate the use of SuperDuper, much as you've done.

With that said, one of the comments notes that you need to partition your USB thumbstick with a GUID partition table, and, while not stated, I would certainly use the HFS+ filesystem for your partition. Did you do it that way? It also appears that you need a flash drive that is at least 8 GB in size.

One more thing to try:

Shut down your mac.
Hold down the option key at bootup.
Do you see your USB flash disk an something to boot off of?
- If so, you can select it to boot off of it. (You can also hold down Command-V while doing so to boot in verbose mode; it should give you white text on a black background, logging details of what the computer is doing. If you can get that far and it gets stuck, can you share some of what it says, especially anything near the end or anything that looks suspicious like it may have gotten stuck there?)
- If not, it means that your computer doesn't recognize the drive as something it can boot off of.

Here are a couple of more thoughts:

if you have another Mac, buy yourself a firewire cable, boot the second Mac into target disk mode (hold down 'T' at bootup), insert the DVD into it, and you should be able to boot off of the DVD through the other computer.
borrow a USB DVD drive, and boot off of it.
just a wild thought that I'll throw out, but would be more trouble than it is worth for one computer, and it well beyond the scope of what you really want to do. You could set up NetBooting to send an image file for the computer to boot up from over the network. Buy Mac OS X Server from the App Store if you are using Lion (and download Apple's server admin tools so you can run Server Admin), or try out JAMF software's NetBoot Appliance (that will run as a virtual machine, under OS X, Windows, or Linux), and (hand waving here; you might be able to use "System Imaging Utility" in the server tools) set up a NetBoot image of your installer DVD to install the software.

MacBook – copy a bootable El Capitan installer USB stick to a hard drive partition and install from there

No, you can't just copy the files from one to another drive and assume that the new drive will install El Capitan. There is a special command to create such an install drive, called createinstallmedia which runs in Terminal

Download the OS X installer from the Mac App Store. Quit the installer if it opens automatically after downloading. The installer will be in your Applications folder.
Mount your USB flash drive or other volume. You could also use a secondary internal partition.
Open the Terminal app, which is in the Utilities folder of your Applications folder.
Use the createinstallmedia command in Terminal to create the bootable installer.

If you're creating from an existing El Capitan machine, please use the following command.

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

I haven't personally tried doing this with an external hard drive instead of a thumb drive. But I don't see any reason for it not to work.

For more details and how to configure the above command (it depends on the OS you're installing to the USB drive from), please refer to https://support.apple.com/en-gb/HT201372

Best Answer

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Creating bootable USB for OS X Leopard

MacBook – copy a bootable El Capitan installer USB stick to a hard drive partition and install from there

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