I have a Mac Min (4.1) plugged into the 42″ thought-killing machine in our living room. Attached is a 1TB external drive, handy for tunes, torrents, etc. In quiet moments, I would notice the drive spin up, even when the mini was asleep, probably every 15-20 minutes or so. Strange? No! This is a common problem with OSX. I did some research. I wasn’t crazy with the suggestion of keeping the drive awake permanently as a solution. That’s better than cycling, but didn’t solve the mystery of why it was spinning up. Somewhere along the way, I came across the suggestion that Spotlight could be indexing the drive. You can disable indexing in System Preferences > Spotlight > Privacy, by adding resources you want to blacklist. I’ve tried this, and it seemed to be the fix for me. I also found another post that describes how to set the fseventsd process to not log activity on external drives. I might try that next, if the Privacy setting isn’t enough.
Now sleep!