PSA: Expired Certificate Means You Need to Remake Any OS X Bootable USB Drives
Making a bootable USB stick is the easiest (and fastest) way to install the latest version of OS X on multiple machines. However, if you still have one, TidBits found that the expired certificate file means they won’t work anymore.
A certificate is required for every app in the Mac App Store, and it turns out that all of the different OS X installers had a certificate that expired in February 2016. Almost all the apps affected by this have already been updated, so if you download something now it won’t have any problems. However, if you are like me and want to have a bootable installation disc or USB stick for troubleshooting, then you will need to download the installer again and remake your USB stick.
Previously downloaded OS X installers no longer work | TidBITS