System shutting down (sometimes) after scheduled backup

[jjj]

But would a failure to respond at the end of a backup make the system shut down (i.e. the whole Mac)?

In answer to your question, no antivirus or the like is running. The scheduled update seems to have run fine. And I've just noticed that the message "kernel[0]: jnl: close: flushing the buffer cache" is the same given when I unmount the mirror partition myself in Disk Utility (such as just after booting).

Speaking of which, is there any possibility that my preference for keeping that partition unmounted, and with other partitions remaining mounted, has anything to do with this?