Last night I left my Mac downloading the update to El Capitan overnight so it would be ready for me to install following the morning.
I was greeted by a rather generic message saying that the download had failed. A bit of digging showed 2.15Gb had been downloaded before the failure. Maybe it was network disruption; I live in the countryside and it’s not unknown for the VDSL to drop out. Nothing to worry about, downloads resume from where they left off , so I tried again.
It failed again.
Activated debug mode for AppStore , cleared cookies, reset application and set the Debug level to 2.
And it failed again at the same point.
Browse of the App Store log file showed NSURLErrorDomain – 1005
Which was helpful in the same way that bricks are to aerodynamics.
Between the internet and my home network is a Squid proxy. I decided to eliminate that as a possible cause.
Several mouse clicks later and El Capitan is downloading without any errors.
Right now I don’t have time to investigate why Squid caused a failure ( max size of cache objects? ) but if others are in a similar situation , switch off the web proxy on your Mac.