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install Mojave after Catalina on external HD

  François-Guillaume RIDEAU 06:35 15 Apr 2020

I am running Catalina 10.15.4 on my iMac. I would like to install latest Mojave on an external HD [to play 32-bit games downloaded on Steam which is impossible on Catalina] I have downloaded the mojave installer on the App store here click here when i run it i got the message

This copy of the "Install macOS" application is too old to be opened on this version of macOS

I googled for a solution but couldn't find anything working for me.


  bremner 07:58 15 Apr 2020

You cannot run the installer to downgrade the Catalina from within Catalina. You need to follow the advice in this link click here

  François-Guillaume RIDEAU 08:48 15 Apr 2020

does that mean that for installing Mojave on an external HD, i must 1) backup the main machine 2) downgrade from Catalina to Mojave on the main machine 3) install Mojave on the external HD 4) upgrade the main machine back to Catalina 5) restore the main machine

looks tedious and perilous !!

  bremner 10:30 15 Apr 2020

Sorry but I am not following what you want.

  • Do you want to downgrade your iMac from Catalina to Mojave? or

  • Install Mojave on to an external drive so you can choose which
    installation to boot from?

In either instance it is always important to have a Time Machine back up of your system in case something goes catastrophically wrong at any time.

  François-Guillaume RIDEAU 10:32 15 Apr 2020

sorry i wasn't clear

I want to install Mojave to an external drive so i can choose which installation to boot from my system is currently on Catalina, and i have only a recent backup of everything on Catalina. no backup from Mojave's days

  bremner 11:49 15 Apr 2020

OK I just got a little confused

Try the advice on this link click here on how to install on an external drive

  appltechie 13:29 29 May 2020

First, make sure your HD holds enough memory - you’ll need at least 32GB, but I’m assuming you’re already at it since you’ve got Catalina. Connect the drive to your Mac. You can get the installer for Mojave via this link. When the software finishes downloading, don’t click on “Install” - you don’t want it to be on your Mac. Next, you need to ensure your drive has the correct format. Go to Applications → Utilities → Disk Utility. Choose your external drive from the sidebar menu on the left. Click on “Erase”. For format, choose “MacOS Extended (Journaled). For scheme, choose “GUID Partition Map”. Click on “Erase” again. Note that this will erase the disk’s contents, so make sure that you’ve backed up everything you need. Now you can install Mojave. Go to Applications and locate the installer. Open it and go past the license agreement. Then you’ll see a page showing the disk it’ll be installed on - it’s set on your Mac’s HD by default. Click on “Show All Disks” - you’ll see your external HD. Click on “Install”. To run Mojave on your external HD, go to Apple Menu → System Preferences → Startup Disk. Select your HD - it will be used to startup your Mac instead of the computer’s own drive. Restart your Mac. Hope this helps!

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