I have a question about downgrading OSX from Catalina to Mojave.
March 26, 2020 10:53 AM   Subscribe

I want to downgrade my MacBook from Catalina to Mojave. I have downloaded the Mojave installer application, "Insall macOS Mojave" (6.05GB). I clone my hard drive nightly to an external drive using Carbon Copy Cloner. So my plan is to just reinstall Mojave on my computer and then copy over everything I need from the external drive. But...

I see online that you're supposed to put the Mojave installer file on a flash drive. But I don't have a flash drive. Can I put in on my external drive that I use for cloning? Will it erase everything from my external drive or do something else bad or not work at all?
posted by Tin Man to Computers & Internet (4 answers total) 5 users marked this as a favorite
Best answer: It needs to be on a flash drive because it has to be bootable media like the olden days with bootable CDs. The drive needs to be formatted in a particular way so the installer can run.

Flash drives are cheap and plentiful; it will be far less effort to pick one up (even in these locked-down times) than to try to do anything else.
posted by jozxyqk at 11:14 AM on March 26, 2020

Best answer: The createinstallmedia command line option to the installer will erase the target device. Do not use it on your external backup drive.
posted by fedward at 11:28 AM on March 26, 2020

Best answer: You need to have another external drive aside from the one that your Carbon Copy Cloner backups are on.

This is because the createinstallmedia command will erase the entire external drive. This would happen even if you are attempting to create it in a separate partition from your CCC backup.
posted by theory at 11:30 AM on March 26, 2020

Alternatively, you could create a small partition (~10gb, to be safe) directly on your MacBook's drive, put the installer on there (be sure to use the "createmedia" command, or download the install creator tool, and point it specifically to the empty partition).

Once the installer is properly placed on the partition within your computer, restart and press ALT/OPT while restarting, and it should show you the installer disk to boot from. Boot from there, erase the main hard drive from within Disk Utility from the Recovery partition, then install.

Once installation is complete, delete the installer partition from the main disk.
posted by dubious_dude at 11:32 AM on March 26, 2020

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