Once you finish the install, jump ahead to Post Install: Configure macOS Catalina. You should read through the Notes and Information sections, and then pick which of the two types of clean installs you with to perform. I’ve organized this guide into four sections: If it does have an OS on it, you’ll need to erase the drive first to ensure a clean installation. The second method has fewer steps, and erasing the target drive is optional, as long as it doesn’t contain any version of the Mac OS. The two methods are similar, though the first, installing on a startup drive, requires a number of extra steps including creating a bootable installer and erasing all data from the startup drive. We’re going to look at two types of clean installs: installing on the current startup drive and installing on a drive that isn’t the startup and doesn’t contain an operating system. So, starting from scratch may be the better choice. You can restore your user data to a clean install but if you’re going to the effort of wiping your startup drive, you’ve probably been having some type of issue that could come back if you restore from the old data. It’s like the day you turned on your Mac for the first time. The downside to a clean install is that not only is the system a new, fresh copy, but your user data is new as well or a better way to put it: all your data is gone, erased.