Hard Disks Do Not Turn Off After Your Computer Has Been Idle
When your Windows 2000/XP/Vista-based computer has been idle for an amount of time that is sufficient to make your hard disk turn off, your hard disk may not turn off. For example, even though your Turn off hard disk setting is set to 3 minutes, your hard disk might not turn off until your computer has been on and idle for 15 to 20 minutes.
For information about disk activity and the MacOS see Apple support article 303698 "Mac OS X: Why your Mac might not sleep or stay in sleep mode".
This behavior can be caused if any disk activity occurs, whether it is caused by user input or system tasks. Disk activity of any type resets the hard disk idle timer, and this can increase the amount of time it takes for your hard disk to turn off.
Windows 2000/XP/Vista/7 is designed to automatically perform maintenance tasks to improve performance and reliability. When your computer is on AC power and is idle, system maintenance tasks may be able to run for a few minutes. These system maintenance tasks include disk-layout optimization to improve performance and preparing automatic system restoration points to increase reliability. Typically, these tasks occur the first time you leave your computer idle after you start it. These tasks ensure that the system maintains its performance and reliability even after long use. Because these tasks involve reading and writing to the hard disk, the hard disk idle time is reset regularly during this maintenance period.
There are other items that can reset the hard disk idle timer. These items can include:
Windows Update checking the system state.
Network detection such as DHCP or Autonet.
Third-party services such as quota software or an antivirus program.
The loading of services or drivers.
When these items either read from or write to the hard disk, the hard disk idle timer is reset. This behavior is by design.
You can use the Power Options tool in Control Panel to configure your computer to turn off hard disks after your computer has been idle for a specified amount of time.