Since one or two months ago, my laptop with ubuntu 12.10x64 hangs during shutdown. There's just a black screen, no harddisk activity, it justs sits there. I feel it happens near the end of a normal shutdown cycle, judging from time elapsed.

Only resolution is hard power-off (holding power key for several seconds) - neither Ctrl-Alt-Backspace (which normally works, I have it switched on in Settings) nor Ctrl-Alt-Del help, nor can I get into a text console with Ctrl-Alt-F1.

How do I diagnose that? It does not happen on every shutdown, but often enough to be annoying (as it forces a diskcheck on reboot, of course).

Which log files can tell me what's going wrong? More or less blindly searching in the sys log viewer reminds me of needles in a haystack.

I imagine I could pinpoint the time it happens via log timestamps, but how do I read timestamps like e.g. in kern.log: [36169.146130]? How do I get this human-readable?

Lastly, a bit of a btw, I've never managed to execute an Alt-Sysrq-REISUB to force a cleaner shutdown, but that's probably because I'm on a laptop, PrintScr is only accessible via an Fn key, and I'm not sure of the exact key sequence I need. Is there a "harmless" but user-visible magic command I could practice this with (during normal operation), so I can find out how to do a REISUB next time?

Thanks all! Comment if you need additional data...