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.

Mac Os Catalina 10.15 Free Download

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.)

10.15.7 is the current release. Launch the Mac App Store on Catalina, and then download macOS Catalina. It should drop into your /Applications folder, where you can reference it from the USB creation documentation. Cancel any attempts by the download to run the installer.
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.
Downloads; Operating Systems; macOS Catalina 10.15.7 macOS Catalina gives you more of everything you love about Mac. Experience music, TV, and podcasts in three all-new Mac apps.

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

The good news is that if you have macOS 10.14 Mojave installed on your Mac or MacBook, then you'll be able to run macOS 10.15 Catalina. Here's the full list of macOS Catalina-compatible devices.

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:

Plug the USB flash drive into your Mac.
Launch the “Disk Utility” application.
On the left-hand pane, select the USB drive (not the partition under it, if any).
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.
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).
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:

# The --volume value is the mounted USB flash drive partition; in this case, named /Volumes/Catalina
sudo/Applications/Install macOS Catalina.app/Contents/Resources/createinstallmedia --volume/Volumes/Catalina
# You will be prompted for your user's administrative password.

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:

Erasing disk: 0%... 10%... 20%... 30%... 100%
Copying to disk: 0%... 10%... 20%... 30%... 40%... 50%... 60%... 70%... 80%... 90%... 100%
Making disk bootable...
Copying boot files...
Install media now available at '/Volumes/Install macOS Catalina'

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

Mac Os 10.15 Installer Download Windows 7

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:

Insert the USB flash drive.
While holding the “option” key down, turn on the Mac to display the Startup Manager.
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.
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:

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.
The macOS Recovery screen contains a top-level “macOS Utilities” menu.
Click on the “Utilities” menu and select “Startup Security Utility”. Input your administrator password when prompted.
In the Startup Security Utility, under “Allowed Boot Media”, select the “Allow booting from external or removable media” option.
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.)
Quit the macOS Utilities to restart the Mac.
On restart, follow the directions in the previous section to boot from the USB flash drive.

Erase the Internal Hard Drive