This is a sort of 'to-whom-it-may-concern post, as I think I have identified the cause and solution to a problem that started with El Capitan on my machines (1). The symptoms are: UserEventAgent, cfprefsd, and mds_stores all use unusually high amounts of CPU time. There is a constant reading/writing to disk at about 1-2 MB/sec. This goes on for hours and hours, far longer than it would take for Spotlight to index the entire disk. The system log contains many messages of the form: com.apple.CDScheduler[pid]: *** LOG MESSAGE QUOTA EXCEEDED... This event gets triggered seemingly randomly, sometimes by Time Machine starting up. Turning off Time Machine reduces but does not eliminate the problem. Turning off Spotlight, either by command-line or excluding whole drives in System Preferences will make the problem go away. Unfortunately, Spotlight is a useful thing, so: The solution appears to be to identify and exclude those directories which contain files in oddball formats which get updated a lot. That sounds a bit vague, but in my case excluding directories like the following has eliminated the problem: Usually in Library/Application Support or Documents, directories belonging to: Parallels Steam VMWare also the download directories of things like bittorrent/usenet programs. I hope this information is useful to someone, A. (1) I think I have solved this problem. Creating this thread will surely cause my computer to start acting up if I have not actually fixed it. Messing with Spotlight improved this situation in that it dramatically reduced the frequency of the problem. It did not eliminate it. I threw in the towel and did a re-install over my existing El Capitan installation. The reinstall got rid of the problem for a while, but it eventually came back. I've given up trying to solve it - I have not found any useful logs. There is a utility called 'mddiagnose' which collects all kinds of data about what is going on with Spotlight, but it points to no smoking gun. Running it stops the runaway CPU use. I have given up trying to solve this for now. I have written a script that monitors UserEventAgent and shoots mds_stores whenever UEA CPU gets too high. Happens about once a day :-( A.