Apple has released their next iteration of OS X - called El Capitan. This version is supposed to be more polished and tested than previous versions, though it comes with some caveats.
All is fine when your operating system is running smoothly and you have no complaints. Apple are big pushers of upgrades to their system and many people would have noticed notifications advising a software update is available practically every time your mac is started. If you have an earlier version of OS X, the App Store would be the big promoter with a large banner offering the free upgrade to El Capitan. So what happens when you upgrade the software and your mac no longer works?
Problems with El Capitan
I had a client last week with the following problem, the upgrade to El Capitan went smoothly enough, and on restarting the progress indicator got stuck on 100% for hours. We would normally suggest reinstalling OS X again, which would involve using Internet recovery to download and reinstall the latest version, but again the progress bar would get to 100% and stall in an endless cycle.
At that stage the options were to try going back to Yosemite 10.10 as that was working perfectly, but on trying to install Yosemite, the system advised you cannot downgrade to an earlier version. Next we tried cloning the drive, fresh installing Yosemite on a formatted drive and using system migration to draw the client data back to Yosemite from the cloned drive. Unfortunately this feature is also disabled - you are not allowed to downgrade. The only solution was to use Time Machine, but the client didn't have a recent enough backup due to a separate issue with Time Machine having not backed up for months, so that wasn't an option.
In order to make a new Time Machine backup, we would need to be able to boot into OS X - which was not currently possible so the client was in quite a state at this stage.
Resolution to the failed El Capitan upgrade
The final diagnosis was that an incompatible kernel extension from a printer driver was the root cause of the problem. After completely removing any offending kexts, launch agents and launch daemons, OS X loaded normally.
The lesson learned from the experience is that the only way is up with El Capitan. You cannot easily downgrade the OS X version unless you have a Time Machine backup - this is now the only way. Also whilst every precaution can be taken with these major OS X updates, no guarantees can be made as to the compatibility of your ancillary software and every effort must be made by you prior to upgrading to ensure maximum chance of success.
It is now difficult to unscramble the failed OS X installation egg - thanks El Capitan…
Happy Computing :-)
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