Mac OS X has an option to keep hard drives spinning for quick file access, or to save power and reduce noise by spinning them down when they're not in use. This can be convenient, but is rather limiting for users who would like to customize when their disks should spin down.
In System preferences, hard drives can be managed in the "Energy Saver" pane by selecting or deselecting the "Put the hard disk(s) to sleep when possible" option.
When this setting is enabled, the system changes the power management system to turn off the disks after 10 minutes of use. When the option is unchecked, the disks are set to never turn off. Despite the limits in the control panel, this behavior can be changed manually, by using the "pmset" command in Terminal. The command alters the power management settings, giving users fine-tune control of the disk sleep settings. It also can set the display and system sleep times (though, the sliders work pretty well for those), and what behaviors will cause the computer to sleep and wake (lid opening, wake-on-LAN magic packet via Ethernet, and so on). For specific information on using this command, review its manual page on it by typing "man pmset" in the Terminal.
To adjust the disk's sleep time, enter the following command in the Terminal (Note: for 10.4 users, "disksleep" will be "spindown"):
sudo pmset -a disksleep VALUE
In this command, VALUE is the number of minutes after which the drives will spin down (with "0" being "never"). The "-a" flag applies the setting to all power configurations (battery, wall power, and UPS). To specify only one of these configurations, change the "-a" (indicating "All") to one of the following: "-b" for battery, "-c" for charger or wall power, "-u" for UPS. For instance, if I want to set the disks on my laptop to spin down after 20 minutes for all power configurations, I would enter the following in the terminal:
Setting the disksleep setting to a lower number than the default of 10 may help conserve battery life if you do not access the hard drive (saving or opening items); however, if you set it too low and frequently save items, the computer will have to constantly spin up the drive--which may even lower battery life. Additionally, the system will have to wait for the drive to spin up before it can access it, which can cause frequent pauses during the saving and opening of files if the disks are set to be spun down all the time. This would be more problematic on wall-powered computers than on battery-powered laptops. The optimal setting will depend on your uses, and while most people might benefit from a reduction to 3-5 minutes from the default of 10, setting it to 1 or 2 minutes may be too low of a value.