This update failed to install when "Preparing" installation on iMac 3.8 GHz 8-Core Intel Core i7. Why?

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Safe Mode will also eliminate Third Party Software, extensions and drives from loading 

3 - Does the issue present in this mode ?

Download the Application Etrecheck directly from the Developer.

The application is free or paid from added features. 

It will take a Snap Shot -  both the hardware and software.

 The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - >>>> using the Additional Text Icon ( 3rd Icon to last ) <<<<

We can have a look at the report for possible issues and may have possible suggestions to resolve the issues.