If you have the hard disks set to be turned off after X amount of minutes of being inactive, then 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 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:
- Paging operations.
- Windows Update checking the system state.
- Event logging.
- Network detection such as DHCP or Autonet.
- Third-party services such as quota software or an antivirus program.
- Scheduled tasks.
- 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.
For more information see: Hard disks do not turn off after your computer has been idle