Bootable external drive for Catalina-older iMac

I wiped an old iMac that used Catalina Mac OS and the OS was wiped too. I would like to reinstall, but am having trouble getting an external hard drive with that OS to boot and install. What is the best way to:

  1. Create a bootable external 30 GB USB drive
  2. Get the external drive to boot as the startup,
  3. Install Catalina to the iMac so I have a clean computer to give away to a friend?
    Happy Thanksgiving to all.

Many thanks,

I would use Recovery Mode to re-image the iMac. Start it in Recovery mode by pressing and holding one of the following key combinations immediately after power-on (before macOS loads):

  • CMD-R. Start from the Recovery volume on your Mac’s internal storage (assuming it still exists).
  • OPTION-CMD-R. Internet Recovery. You can use this to install the latest version of macOS that’s compatible with the hardware.
  • OPTION-SHIFT-CMD-R. Internet Recovery. You can use this to reinstall the version that came with the hardware, or the closest version that’s still available.

See also: Use macOS Recovery on an Intel-based Mac - Apple Support

When in Recovery mode, you can definitely reinstall macOS on the internal storage. I think you can also use it to erase and install macOS on an external volume as well.

To boot from an external drive…

If your Mac has a T2 chip, then you need to use the Startup Security utility in order to enable booting from external devices. To do this:

  • Boot into Recovery mode

  • Run the Startup Security utility

  • Select the appropriate security level:

    • Full security will only boot versions of macOS that Apple currently trusts. Which means old versions might stop working if they’re not kept up to date.
    • Medium security will boot any version of macOS that Apple has ever trusted in the past. This is what I recommend, especially for an older computer that may not be getting regular updates.
    • No security. This is really only going to matter if you want to boot something other than macOS (or a version of macOS not meant for use on your hardware). Don’t use this unless you have to.
  • Allow booting from external/removable media. I recommend everybody enable this unless you have a good reason not to (e.g. if it’s a computer in a publicly-accessible location), since it is needed for booting from system backups and installation media as well.

See also: About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

Once the computer is able to boot from external volumes, you can do one of the following:

  • Choose your external volume it as the startup disk. You can do this from macOS system settings or from Recovery mode. It will write data to the Mac’s NVRAM so the selected volume will be the default startup volume until you change it.

    If the selected drive isn’t attached when you boot, it will wait a while (in case it’s taking a bit of time to become ready) and then will try to locate and start from another available bootable volume.

  • Hold down OPTION while booting the Mac. You should see a list of bootable volumes, and should include any bootable USB devices. Select one and click Continue to boot from it. This will not write anything to NVRAM, so your next reboot will go back to its default boot device (as selected via Startup Disk).

See also:

1 Like

Thanks for the suggestions. Unfortunately Recovery mode did not work for me because after a previous erasing, I had formatted my boot drive incorrectly for this iMac.

Below is an update with a solution I did not know about to reformat my boot drive when it had been formatted APFS while the new to be installed OS required Extended (Journaled) format for my older iMac.

The machine in question is an older Intel iMac. I troubleshot why using Recovery mode was not working for me.

When I booted into Recovery Mode and selected the install OS option (High Sierra), I got an error message that the boot volume HD was formatted APFS and I needed the boot volume to be formatted as Extended (journaled). I must have formatted APFS after I erased it.

It was not at all apparent to me how to reformat, even when I quit the Install OS app and selected Disk Utility and Erase. DU offered some format options, but they were all limited to flavors of APFS. Not what I needed and it appeared to be a dead end. Sort of a Catch 22.

After researching further, I learned that I needed to get back into DU while in Recovery mode. I then needed to choose the boot drive and from the menu select Edit, then choose Delete APFS Volume and let the volume get deleted.

Next step is to quit DU.

Once out of DU with the boot volume now “deleted,” open DU again and select the boot volume (it had been renamed). This time, Extended (Journaled) and the many other format options were possible. I selected Extended (Journaled), DU did its magic and the boot volume was reformatted. I had the option to rename it Macintosh HD. I then quit DU.

Once the boot volume was reformatted, I was able to use recovery mode to install High Sierra on the clean boot drive.

The iMac appears to be ready to be given away. The new owner will be able to set up OS following the usual initial prompts.