Every time Apple release new major versions of OS X I create a video showing you how to create a boot disk from the downloaded installer. Most of the time once you've checked the boot disk works you can safely leave it in a drawer until the fateful day you need it! However, an issue with a certificate means that if you don't do something about it now that carefully made boot disk will not work when you need it most.
Apple had an issue with the inner working of the App Store back in late 2015. The technical explanation was a certificate update that had unfortunate knock on effects for end users. The practical effect of it was to annoy users endlessly with incomprehensible error messages. Although assuring users the issue was resolved Apple again had certificate problems in February 2016. Once again the issues were resolved by the issuing of a new certificate.
Sadly one of the effects of this new certificate is that OS X installers download prior to the issuing of the new certificate and boot disks made from them will no longer work.
What Should You Do?
The good news is you have options and assuming your system is up and running you can take action now!
The Best Option
The best way to proceed is to:
- Ensure all old copies of the installers have been removed from both the internal and mounted external drives
- Navigate to the Purchased Tab of the Mac App Store and locate the installer required
- Download a new copy of the installer for your system
If you see the "Downloaded" message next to the installer it is likely there is an old copy of the installer somewhere on your system.
If you are sure you have removed all copies of the installer then it may also be worth logging out of the Mac App Store and back in again and even rebooting your Mac.
The Emergency Option
There is an emergency option if you are in the middle of attempting to install OS X and don't have the opportunity to re-create your install disk. As long as you're comfortable in the Terminal there a procedure to assist. In essence it involves setting the system date back to before the certificate expired. Read all the details at TidBITS.com.
Installing OS X
Once you have a new copy of the OS X installer you need by far the easiest way to create a bootable installation disk for OS X is using DiskMaker X from http://diskmakerx.com/. DiskMaker X is a free app created with AppleScript that can be used to create a bootable install disk.
Here are direct links to tutorials for each version of OS X: