Although the process of installing the macOS has become much easier and more intuitive in recent years, many people still like to keep a system boot disk This is an external storage device, such as a USB stick, with a macOS installer ready to stick on your machine if something goes wrong and you need an instant solution.
Over the past few days, however, many users who have had to power up their boot disks have come across an error: while trying to boot the computer from the external drive, they received error messages stating that the file was damaged and could not be used to install macOS. The problem has occurred with installers of various versions of the system since the Yosemite at the Mojave.
If you were one of the users hit, fear not: the problem, as explained in a new support page published by Apple, has simply to do with an expired certificate for these installers.
How to solve?
To solve the problem, you will need to delete your current installer and create a new one from a new system installation file of your choice. Below are the download links for installers for the latest six versions of macOS:
If you have created a macOS installer before, you probably know what you are doing; Still, if you need a little help, just follow the steps outlined in the ?Creating a Boot Disk? section of this article, replacing, of course, references to macOS Catalina with the version of the system you choose.