sleep mode

does the hard drive on my iMac continue to run when in sleep mode? Is overnight power down recommended to prolong the life of the (mechanical) drive?

When your Mac sleeps, the HD is shut off.

Actually, your HD will stop even if the Mac is on (assuming you have that option checked in System Preferences) when it is not being accessed.

Sleeping your Mac will ultimately put less wear and tear on the hard disk than starting it up every day, since it doesn't have to load the whole OS every time. But I doubt that a daily start-up sequence (or even three) would have a significant impact on your HD's life, anyway.

When your iMac sleeps, the hard drive likewise spins down if you have set it to do so:

System Preferences -> Energy Saver -> Sleep Tab. Place check mark in box that says "Put the hard disk(s) to sleep when possible".

As for your second question about shutting down.... There's pro and con. I personally shut my system down whenever I'm going to be away for awhile or overnight. Severe weather here in Texas can occur very quickly and sometimes without much warning. I don't take chances.

Regards.

As i have a UPS so i just hit Command Option Eject each night and then it is all go next morning. Then every 3 - 4 days i do a shut down/reload cycle to keep things sweet.

An UPS is great for folks who have a desktop machine and don't want to lose data during a power interruption. But an UPS won't protect against lightning strikes or near strikes.

Don't know if you get the kind of severe weather in NZ that we get in Texas, but around here thunderstorms come with hail, lightning, and high winds. Not to mention an occasional Tornado! Just had my roof replaced on my home due to a hailstorm we experienced in late April. Sounded like 20 guys on my roof beating it with hammers.

Regards.

Part of the reason for this was that it was a reasonable initial defence against most power issues including lightning. But then we seldom have strikes around here so...... fingers crossed - the ultimate protection.