Unexplained shutdowns
QAlthough my Mac mini seems to have been running sweetly with El Capitan, a couple of times recently it has shut down of its own accord, but the shutdowns haven’t coincided with any interruption to the mains power supply. Why could that be?
AThese are almost certainly the result of kernel panics, in which the heart of OS X has become so badly damaged that its only option is to force your mini to shut down. As you’re still running El Capitan, you can check whether a recent shutdown is the result of a kernel panic, by browsing its logs in Console.
Search its All Messages view to find BOOT_TIME in the first instance, which shows the last time it started up after shutting down like that. From there, show all log entries again and scroll backward in time to locate its last entries before starting up, and forward in time to look for ‘Previous shutdown cause’. The latter should give a code which you can interpret from eclecticlight.co/2017/02/28/ mac-shutdown-and-sleep-cause-codes.
Next, run Apple’s hardware Diagnostics to ensure there isn’t a hardware problem lurking, such as faulty memory. Then start up in Recovery mode, open Disk Utility, and run First Aid on your startup disk to ensure it’s OK. Those are detailed in eclecticlight. co/2017/03/22/running-hardware-diagnosticsor-aht and eclecticlight.co/2016/08/25/ recovery-mode-and-its-tools.
If those are good, suspect a rogue app, or the possibility it’s the longstanding tendency to instability in El Capitan. This is a very good reason for upgrading now to Sierra, which on most Macs is considerably less prone to such kernel panics.