OS X updater claims installation is successful, but new OS is not installed I've got a Mac running 10.10.5 Yosemite, trying to upgrade to 10.11 El Capitan. When I run the installer, it claims installation requires 7 Mb of disk space, it installs some files, and ends with "Installation successful".  

But the OS is not updated: after a reboot 'About this Mac' still shows it running Yosemite. 

The same thing happens on this Mac when running the 10.12 Sierra installer. The goal is to get to Mojave, but that installer won't run at all so I'm trying incremental updates. 

In both cases I made sure to download the full (~6 Gb) installer beforehand (from the app store), and I'm running the installer from the mounted disk image.

What could cause this error, and how do I get rid of it?
This is the information I missed. I had downloaded a .dmg file, this installs the 'Install El Capitan' app (containing the full 5 GB installer) in the Applications folder. I was expecting it to install the OS instead.

The linked instructions show how to create a bootable installer on an external drive from the .dmg.


connect an external drive
Use the 'createinstallmedia' command in Terminal

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


You can then boot the machine from the external drive and use it to update the OS on your normal boot volume. 

The drawback of this approach is that it erases the external drive.