My wife has a new Mac Pro running Leopard of course, and we decided to change our backup strategy to use TimeMachine. For her, this is a great solution, because then she can very easily jump back to older versions of files she worked on. Our previous backup, which was a NAS I had rigged up, allowed for the same thing, but it was difficult to deal with and prone to disappearing off the network at the most inconvenient times.
So, since we had two external USB2 drives, it was just a matter of plugging 1 into her machine, formatting it, and turning time machine on. All done. However, since then that external drive would spin up every 5 minutes at times, and other times, never would. It wasn’t a big deal, not loud or anything, but…
You know those noises that are low enough to be just barely heard? You know how they can be more annoying than construction outside your window? This noise was exactly that. vweeeeeeeeeewwwwwwpopbzzzzzzzzrrrrrrrshewww, but just on the edge of hearing. Also, it was causing some slight slowdown at times, which was surprising for how fast this machine is. So, how do you fix it?
Well, it’s spotlight’s fault it seems. Since this external drive is purely a timemachine drive, there is no reason at all to index it. But spotlight does it anyway, just because it can I guess. It’s a really easy fix. Go to your system settings, and open the spotlight settings. Click on the Privacy tab, and then add the external drive to the list. Now spotlight will stop indexing that drive and the noise/slowdown will go away. quick an easy.
Though… As I was typing this, the damn drive just spun up. Of course that may have actually been timemachine doing a backup since its on the hour… I will post an update if it turns out not to have fixed it.
update: no joy. it didn’t fix it. gah!

Did you ever discover a fix for this issue?
I have not. It went away with an upgrade to snow leopard, which just avoided the problem i think. Plus we stopped using an external like that a little while later.