• I am trying to upgrade an older iMac (2008) from OS 10.6.8 to El Capitan for a disabled client who really needs to use a Mac instead of a PC!
• EveryMac says it can handle El Capitan if I upgrade RAM. It has 4GB now.
• I have an Installer file from my own Mac (always keep them when I upgrade). I made a bootable flash installer that mounts and seems to have the proper files, but it won't show up on either Mac on restart holding option key; nor in the Startup Disk list. So, I figured writing to the flash drive must have failed.
• I partitioned an external hard drive and redid this:

  'sudo /Applications/Install OS X El Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install OS X El Capitan.app'

• It seems to work (again) in Finder AND it shows up as a boot option on restart (holding option key), but when I select Install OS X I get an error that the installer cannot be verified. I can't get it to work.
• So, I figured my installer must have gone bad, even though I successfully upgraded my own Mac with it, and have since upgraded mine to Sierra.
• Using the App Store I can get to El Capitan, but it will not allow me to download it on my Mac because the system is too new, and I don't want to download it to the old Mac because I want to use an external drive to do a clean install; not an upgrade on the old one.
• Any advice?

IMAC (RETINA 5K, 27-INCH, LATE 2015), macOS Sierra (10.12.3)

Posted on Feb 28, 2017 9:53 PM

1. The Mac OS X El Capitan 10.11.1 DMG is the twelfth major release of OS X providing different enhancements and many new features. Aug 22, 2018 Download Mac OS X El Capitan (10.11) DMG file for free. Mac OS X El Capitan (10.11) update setup file for free. How to install Mac OS X El Capitan.
2. As explained here, OS X 10.11 'El Capitan' fails to install because it is too old (the technical reason being that the signing certificate is no longer valid). But you can trick the OS X installer into thinking that it is 2017 and it will happily run without any errors.
3. Mac OS El Capitan Installer by Apple Inc. Publication date 2020-01 Topics Software, Mac OS, Apple, inc. I am uploading Mac OS El Capitan that I downloaded off of.

Mac Os El Capitan Installer

Use the arrow keys to select Boot OS X Install from Install OS X El Capitan (It should be selected by default). Press the spacebar and select Boot Mac OS X in verbose mode. Verbose mode means that you'll be able to see exactly what's going on under the hood as OS X attempts to boot up its installer.