System shutting down (sometimes) after scheduled backup

[jjj]

No panic.log, no panic screen ("You need to restart your computer," etc.). Just the next morning, my Mac ain't on.

I watched a scheduled backup this time. Everything seemed to work. This is the system.log:

Oct 10 06:30:16 slynew diskarbitrationd[42]: disk1s2    hfs      CBADEC29-3F8C-357E-8B45-7D072826D110 Mirror                  /Volumes/Mirror
Oct 10 06:30:33 slynew KernelEventAgent[35]: tid 00000000 received unknown event (256)
Oct 10 06:30:33 slynew KernelEventAgent[35]: tid 00000000 received unknown event (256)
Oct 10 06:40:55 slynew diskarbitrationd[42]: applet [591]:19411 not responding.
Oct 10 06:40:55 slynew kernel[0]: jnl: close: flushing the buffer cache (start 0x176200 end 0x178400)

The mirror partition did unmount (or at least it is unmounted after this), and the backup log shows no complaints.

I guess I'm just going to reschedule this during the day and keep watching it until something happens.