You May Need to Re-download Your OS X Installer
One of the recommendations I make in my Take Control of Upgrading to… books could potentially cause a headache unless you know how to avoid it. I tell you to save a copy of the OS X installer that you’ve downloaded so that, if the occasion should ever arise when you need to reinstall it or put it on another Mac, you don’t have to download it again. Saving the installer requires deliberate action, because if you leave it in /Applications and run it from there, OS X deletes the installer app once your new version of OS X is in place. (Some people go a step further and create a bootable volume, such as a USB flash drive, containing the installer, which makes it easier to install on multiple Macs.)
Unfortunately, if you followed my advice and kept a copy of your installer prior to February 14, 2016 (whether or not you turned it into a bootable volume), you’ll find that the installer no longer works the next time you try to run it, because the certificate that Apple used to sign the installer (a crucial security step) expired on that day. Josh Centers explains the whole thing in the TidBITS article Previously Downloaded OS X Installers No Longer Work.
If you’ve already saved the installer but don’t need it yet, you can delete it and download a fresh copy (now updated with a new certificate); that way you’ll be ready if and when you need it. If you created a bootable installer volume, you’ll need to erase it and follow the instructions in my book to recreate it.
Should you find yourself suddenly in need to reinstall OS X but not have an updated installer handy, there are ways (mentioned in Josh’s article) to trick OS X into running the old installer, but they’re not ideal, and I recommend planning ahead for best results.
This issue appears to affect every version of OS X from OS X 10.11 El Capitan back to 10.7 Lion, although a few users have reported that their previously downloaded installers continue to work (the reasons for which are unclear).
Posted by Joe Kissell (Permalink)