While, for the most part, it’s been reinstated since, with developers being urged to update their certificates to prevent their services from becoming unusable, there remains but one exception. Any OS X installer obtained prior to Valentine’s Day will be virtually unusable from here on out.
That includes installers for El Capitan, Yosemite, Mavericks, Mountain Lion, and Lion, whether they were acquired from the App Store or on a USB install disk.
Nevertheless, if you need to reinstall an older version of OS X for whatever reason, you can do so on the App Store for free. Keep in mind, however, that for whatever version of the operating system you’re trying to get, you need to have downloaded it before, as it will only be attainable through your purchase history.
Likewise, it may also be worth mentioning that you can’t even download the installer file for a version of OS X that doesn’t work in conjunction with the computer you’re downloading it on. For instance, as Ars Technica editor Andrew Cunningham writes, “my 2012 iMac can download everything back to Mountain Lion, but it refuses to download Lion.”
Oddly enough, despite the certificate having expired, there’s an eyeroll-inducing fix for anyone looking to use an older installer. The trick? Change the date on your Mac to any date prior to February 14, 2016. As frivolous as that may be, it apparently works, according to Ars.
Alternatively, if you’re stuck in the middle of an OS X install and the expired certificate causes problems, there’s an easy workaround. On the OS X installer page, simply select Utilities, then Terminal, enter the date 0201010116, and press Enter/Return. From there, you can continue the install, presumably without struggle.
