On February 14th, Apple’s Worldwide Developer Certificate – which verifies third-party apps and services – expired, which meant Apple issued a new certificate for developers to use. Developers were advised to update their certificates in order to ensure their Apple Wallet Passes, Mac apps, extensions, and such would continue to work.
One by-product of that, which is of importance to those who regularly troubleshoot and fix Macs, or just reinstall OS X often. (You know who you are..) Any OS X installers that were downloaded from the Mac App Store before February 14 will no longer work. That includes installers for El Capitan, Yosemite, Mavericks, Mountain Lion, etc. If you’ve created a USB install stick using an installer from before Valentines Day, you’ll need to re-download the installer and create a new install stick.
One helpful note: If you do need to reinstall OS X using an old installer, you can set your Mac’s date to sometime before February 14th, and the older installers should work properly. So, you’ve got that going for you. Which is nice.
For more workarounds, and other helpful information about this subject, visit the TidBITS website.
(Via Ars Technica)