When you disable the setting "Put disks to sleep whenever possible" is this a per-user setting or are you setting the preference for the whole system? What I'm getting at is that I'd like my external drive on my workstation to stay spinning when I'm working, but at night I log out and want the drive to spin down. Anybody know?
You could try it yourself if it works, but you could also put the Mac to sleep or power it down, that will spin the HDDs down. As far as I know, the setting in Energy Saver is user dependent.
Energy Saver is not a user level preference. It is system-wide. The plist resides in /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist If you want different behavior, I would create login and logout hooks that change the "Disk Sleep Timer" setting. For details on login and logout hooks see here: http://www.bombich.com/mactips/loginhooks.html