Interesting that Catalina downloaded first time, without issue on the A1398 with new drive and brand new install of High Sierra. Seems a bit like I should blitz the drive on my old A1398 and do a clean fresh install of High Sierra, after all I HAVE just built a bootable USB for it.:P. The first time I reinstalled Catalina, it was a process so I just wanted to get it documented here. Before doing anything, make sure you have a backup of all the important files on the system you are planning on a Catalina install because this is a destructive process to the files and file systems on the root disk and you will ultimately be to.

I purchased a used 2018 Mac mini desktop and wanted to put a fresh installation of the latest macOS 10.15 Catalina on it. Below are the steps I took.

Note: The instructions below are the same for macOS 10.13 High Sierra and macOS 10.14 Mojave.

Download macOS Catalina

You will need a Mac to download macOS Catalina; you can download Catalina from The Mac Go. (The Mac mini came with macOS 10.14 Mojave so I used it to download Catalina.)

Go to the Mac App Store, search for “macOS Catalina”, and download it. It will be saved to the “/Applications/Install macOS Catalina” location. If the installer automatically launches, just close it.

Note: If you plan to run the installer first to update your current macOS version to Catalina, you will want to move the “Install macOS Catalina” application out of the “/Applications” directory; otherwise, the installer will delete itself from the “/Applications” directory when the upgrade completes. For example, you can move the “Install macOS Catalina” application to the “~/Downloads” directory and run it from there.

Format USB Flash Drive

Because macOS Catalina is 8.4GB in size, you will need a 16GB USB flash drive. You don’t need to delete the existing content on the drive because we will blow it all away in a subsequent step.

If the USB flash drive is not already formatted as “Mac OS Extended (Journaled)”, format it like so:

1. Plug the USB flash drive into your Mac.
2. Launch the “Disk Utility” application.
3. On the left-hand pane, select the USB drive (not the partition under it, if any).
4. Click on the “Erase” tab (or button at the top).
   1. Input a name like “Catalina” (this name will be overwritten later).
   2. Select “Mac OS Extended (Journaled)” for “Format”.
   3. If available, select “Master Boot Record” for “Scheme”.
   4. Click the “Erase” button at the bottom. Click the “Erase” button in the warning popup dialog if you get one.
5. The format operation may take several minutes to complete. (USB 2.0 and large capacity drives may take longer.) After the format completes, the partition will be mounted under “/Volumes/Catalina” (or whatever name you selected above).
6. Close the “Disk Utility” application.

Write macOS Catalina to USB Flash Drive

To create a bootable USB macOS Catalina installer, run the “Terminal” application and this command:

Note: If the createinstallmedia command returns a “Failed to start erase of disk due to error (-9999, 0)” error, then your current Mac OS X version does not fully support the createinstallmedia tool. Use a more recent macOS version (at least Mac OS X 10.11 El Capitan) or manually create the USB installation drive using instructions from Bootable USB Flash Drive to Install Mac OS X 10.10 Yosemite.

The “createinstallmedia” program will erase the USB flash drive, create a new partition named “Install macOS Catalina”, and copy the installation files to that partition. The output will look like:

The above process took 20-30 minutes using my Kingston USB 2.0 flash drive.

Boot From USB Flash Drive

Note: Apple’s How to create a bootable installer for macOS page suggest using the “Startup Disk” configuration (under “System Preferences”) to select the USB flash drive as the startup drive. Then on restart, the Mac should boot from the USB flash drive. Unfortunately, the “Startup Disk” did not list my USB flash drive as a startup option, so I was not able to use this method.

To boot a Mac with the USB flash drive:

1. Insert the USB flash drive.
2. While holding the “option” key down, turn on the Mac to display the Startup Manager.
3. You should see one or more icons, one of which should be called “Install macOS Catalina” for the USB flash drive. (The internal hard drive may not be visible if it does not have a valid, bootable partition installed.)
   • Note: If you don’t see the USB flash drive’s “Install macOS Catalina”, try removing and re-inserting the USB flash drive while viewing the Startup Manager screen. The USB flash drive should then appear after a few seconds.
4. Select the “Install macOS Catalina” (with left/right arrow keys) and hit the “return/enter” key to boot from the USB flash drive.

It may take 5-10 minutes to load the installer from the USB flash drive. Sometimes the progress bar may appear to be frozen or the screen will go black for a minute or two. Be patient.

External Startup Disk Not Allowed

On a newer Mac with the T2 security chip, booting from an external drive may be disallowed by default. Attempting to boot from a USB flash drive will result in a “Security settings do not allow this Mac to use an external startup disk” warning message.

To allow booting from the USB flash drive, do the following:

1. Hold down “Command-R” (hold down both the “Command ⌘” and “R” keys) when you start the Mac to launch macOS Recovery. You can release the keys when you see the Apple logo.
   • Tip: An easier alternative is to hold the “Option/Alt” key on boot to launch the Startup Manager. Then just click and release “Command-R” keys to launch macOS Recovery.
2. The macOS Recovery screen contains a top-level “macOS Utilities” menu.
3. Click on the “Utilities” menu and select “Startup Security Utility”. Input your administrator password when prompted.
4. In the Startup Security Utility, under “Allowed Boot Media”, select the “Allow booting from external or removable media” option.
5. If the Mac won’t have access to the Internet (either by wire or wirelessly) when installing macOS Catalina, make sure to select either the “Medium Security” or “No Security” option under the “Secure Boot” section. (You can change the option back to the default “Full Security” afterwards.)
6. Quit the macOS Utilities to restart the Mac.
7. On restart, follow the directions in the previous section to boot from the USB flash drive.

Erase the Internal Hard Drive

When the installer finishes loading, you will see a “macOS Utilities” screen appear. It looks the same as the “macOS Recovery” screen with one difference. In the “macOS Utilities” window, the second option will say “Install macOS” instead of “Reinstall macOS”. In “macOS Recovery”, the “Reinstall macOS” option would just re-install the same version of macOS. The “Install macoS” option will install the macOS Catalina version on the USB flash drive.

Before installing macOS, I wanted a clean slate so I erased the internal hard drive like so:

1. Select the “Disk Utility” option and click the “Continue” button on the bottom to launch the “Disk Utility” application.
2. On the left-hand pane, select the internal hard drive (not the partition under it, if any).
3. Click on the “Erase” button at the top.
   1. Keep the default “Macintosh HD” name or input a new one.
   2. Select “APFS” for “Format”.
   3. Click the “Erase” button at the bottom. The SSD (Solid State Drive) format took less than a minute to complete.
4. Close the “Disk Utility” application.

Install macOS Catalina

Warning: If your Mac has a T2 security chip and you left the “Full Security” option configured in the Startup Security Utility, make sure that Internet access (wired or wireless) is available before starting the macOS Catalina installation. You can configure the wireless network by clicking on the wireless icon at the top-right corner of the macOS Utilities screen.

Back at the “macOS Utilities” window, do the following to start the macOS Catalina installation:

1. Click on the “Install macOS” option and click the “Continue” button.
2. The “macOS Catalina” installer’s splash screen will appear. Click the “Continue” button.
3. Click on the “Agree” button to agree to the license. A popup confirmation window will appear; click on the popup’s “Agree” button.
4. Select the internal hard drive and click the “Install” button.

My Mac mini took 20-30 minutes to complete the macOS Catalina installation. I think there were a couple of reboots in-between. And a few times, the screen would go black for a minute or two. Be patient and give the process an hour or more to complete.

If you run into problems installing macOS Catalina, check the “What Does Giving Up Mean?” section (at the bottom) of Install macOS Sierra Using Bootable USB Flash Drive for possible solutions.

Note: macOS Catalina seems to have an issue with my Dell 24in U2410 monitor. During the initial bootup screen, the monitor shows a blank screen with a message, “The current input timing is not supported by the monitor display”. Thankfully, the monitor works correctly after the bootup completes. And later, this warning message no longer appeared on bootup.

Enjoy your fresh installation of macOS 10.15 Catalina.

Apple released macOS Big Sur on November 12, 2020. macOS Big Sur continues the list of Central California landmarks as well as marks the new operating system for Mac. Those who tried it say that it’s more stable. It also sports a refreshed version of Safari that is 50% faster than Chrome. Here’s how to perform a clean install of macOS Big Sur with pictures and helpful tips along the way.

What is a clean install? (Big Sur or any other macOS)

A clean install means that you install the macOS onto the erased drive.

Why is clean install more preferable?
A clean install is the opposite of macOS upgrade. You format your hard drive, and therefore, it keeps no leftovers from the previous macOS.

• macOS performs faster
• You get rid of apps you don’t use
• Persistent issues may go away

Basic steps to clean install macOS Big Sur

The first step is to clean and back up your Mac to some external source. Then, you’ll need to download macOS Big Sur package to your Applications. Next, you’ll need to create a bootable USB installer (involves another external drive). And lastly, we’ll use Disk Utility to erase your Mac and install macOS Big Sur using the bootable USB device.

1. Clean & Back up your Mac

Cleanup

Cleaning your Mac is generally a healthy practice but since we’re about to back your Mac up, it’s worthwhile to get rid of system junk first. After we’ve cleaned your Mac, you’ll end up with a refreshed system ready for a Time Machine backup.

Time Machine will make a snapshot of your system.But before that we’ll ensure there's no unwanted leftovers on your entire Mac. To clean your machine automatically, you can run CleanMyMac X. This tool is one of the best-known cleaning utilities for Mac, and it’s been notarized by Apple.

Download the free version of CleanMyMac X cleaner here.
Install the app and click on System Junk to perform the scan.

Back up your Mac

Now, you’re ready to back up your computer. There is a tool built just for that purpose, you guessed it, Time Machine. Time Machine needs a storage source onto which it will copy your system.
So, you’ll need an external drive. This is the first external drive that we will use to clean install macOS Big Sur. This is the reserve backup for the worst-case scenario.

• Go to System Preferences > Time Machine.
• Connect an external drive (that’s more spacious than your current system).
• Select this Disk as a backup source in Time Machine.
  

Well done. The preliminary steps are done with, and we’re ready to clean install macOS Big Sur.

2. Download macOS Big Sur installer

macOS Big Sur has been released on November 12, 2020.

Since the public release in the fall of 2020, macOS Big Sur can be downloaded from the Mac App Store. It also can be found in the Software Updates pane in System Preferences.
So you’ll find it in System Preferences > Software Updates.

• Click Upgrade Now
  

Wait for the installer to download onto your drive. You will see the macOS Big Sur intro screen.

Don’t click Continue but for now, close this window by choosing Quit Install.

We only want to make sure that the app called Install macOS Big Sur.App is now firmly sitting in your Applications.

This is the file we’ll use later to run the clean install of macOS Big Sur via Disk Utility.

Now, and only now have we come to clean installing macOS Big Sur. This is only a short stop on the road, so catch your breath, and let’s move along.

3. Create a bootable Big Sur installer

Now, it’s time to grab another USB flash drive and connect it to your Mac. This drive should have at least 16 GB of storage space.

Wipe the drive clean with Disk Utility

1. Pick this drive and click Erase in the top menu.
2. Double-check what you’re erasing.
3. Make sure the USB drive is shown as “Untitled”
4. Under Format, choose: Mac OS Extended (Journaled)

All data on that disk will be lost, so beware.

Go to Terminal (Phew, we’re almost there)

This step will actually create the source disk so we can install Big Sur from bootable USB.

Launch Terminal app from your Launchpad.

Paste in the following command:

sudo /Applications/Install macOS Big Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled

Type in your admin password and press Return and Y to confirm the action.

This command creates a bootable USB installer based on macOS Big Sur.app, which we’ve previously placed in your Applications.

4. Erase your startup drive in Disk Utility

It’s time to wave goodbye to your current configuration. Remember, we’ve already cleaned it up from junk with CleanMyMac X and backed it up onto our first reserve USB drive.

Now, start your Mac in Recovery mode:

• Turn off your Mac and hold down Command + R.
• Choose Disk Utility from the menu.
• Choose your main startup drive and click Erase.

Voila! Your Mac is now formatted. What you can do now (actually, this the only thing you can do now) is to clean install macOS Big Sur 11 from the connected USB installer drive. If it’s not connected, connect the bootable USB installer now.

1. Go back to the original Disk Utility screen
   (If you can’t return to Disk Utility, restart your Mac and hold down the Option key)
2. Click Install macOS and choose to install it from the selected USB drive.
3. Click on “Install macOS”

Install Catalina From Bootable Usb Drive

How do I regularly install macOS Big Sur?

The regular install is done via upgrading your current macOS. That is, you don’t erase your entire disk. When the new macOS Big Sur is officially released, you just open the App Store > Updates.

Install Catalina From Bootable Usb Download

Or even better, go to your System Preferences > Software Updates

How to reinstall macOS Big Sur?

If for some reason your Big Sur macOS doesn’t work as expected, you can reinstall it by sending your Mac into Recovery Mode.

Install Catalina From Bootable Usb File

• Restart your machine and hold down ⌘ + R
• Choose Reinstall macOS

That’s it.

As we’ve just seen, running a clean install of macOS Big Sur could be a crazy rabbit hole. We hope you survived. Now you’ll be rewarded with the best Apple’s operating system up to date. You’ll also have a clean Mac that works at blazing speed with a little extra help from our favorite Mac cleaner — CleanMyMac X. You can get started with a trial version — so download it here.