Under the key "Disk Sleep Timer," you can edit the number of minutes until an idle drive goes to sleep. There are separate settings for AC Power and Battery. Sadly, the setting cannot be set per drive, but at least it prevents my external RAID from powering down after five minutes of inactivity.
[robg adds: This other hint explains how to do this using a program included with the Developer Tools. And, of course, if you don't want them to sleep at all, then just uncheck the 'Put hard disks to sleep when possible' box in the Energy Saver System Preferences panel.]

