I keep a PC running 24x7 as it handles several light (processor-wise and disk-wise) tasks and acts as a jumping point into my network from outside. I'd like to keep it as energy efficient as possible.
I've always wondered about the "Turn off harddrive after XX mins inactivity" type settings. I can imagine on Win 3.1 that this might work well for unattended PCs, but I figured that in more modern OSes there are so many background tasks running and likely touching the disk periodically that this setting is pretty ineffective.
However, I'm more concerned/interested in knowing whether it is dangerous. I can imagine that spinning down the harddrive every 20 mins just to have it get spun right back up again (and maybe worse, spun up while it is spinning down) might be bad for the drive. I've definitely seen some TCO figures from way back about lab machines used only during the day but cheaper to leave them running since the drive failures are increased with spin up/down (and they seem to occur during power on).