Is there anywhere in the log that I can find out the time and how it was powered down ? I only found out that it was powered down when I tried to access it.
The log does show "normal" shutdown and power-up/reboot events, so you should be able to find the reboot event corresponding to when you restarted the NAS after you noticed that it had gone offline. Then, the events logged just prior to that may give a clue as to what happened - did it do a complete power-off or did it hang?
FWIW, while troubleshooting a different issue, I have had my DNS-320 (F/W 2.02) connected via a Trendnet TEG-S5g 5 port gigabit switch (with the "GREENnet" feature to control the power usage). Although it's been running for only a few hours connected to the Trendnet switch, I haven't had any unexpected power-down or reboot events. But, on one occasion, when the hard drive spun-down due to inactivity, the Windows 7 PC with the drive mapped showed a red X against the drive letter as if the network share was no longer available. However, clicking on it, the drive spun up and its contents were shown OK, so I'm putting that down to a Windows glitch, not the NAS.
Regards,
Richard